refactor(css): move document, gallery and image-editor styles to their own file

static/style.css is 51,425 lines and holds every panel's styles, so two
people working on unrelated panels still edit the same file. This is the
first of three steps that give each panel group its own stylesheet. It moves
the document library, image gallery and image editor rules that can move
into static/css/documents-gallery-editor.css.

The new file is loaded eagerly from index.html immediately after style.css,
so the cascade is the concatenation of the two in that order, which is the
order those rules already had. Nothing is deferred and nothing renders
differently.

Rules moved verbatim. A rule stays in style.css when its selector group
covers more than one panel, when the class is shared design language used
from modules outside the panel, or when moving it would flip which of two
equally specific rules wins on an element the markup puts both classes on.
That last rule is what keeps this change small: most panel-prefixed rules on
this branch compete with a shared component rule somewhere later in the file.

Nineteen tests read static/style.css directly and go red the moment a rule
they assert on moves, which says nothing about the page. They now go through
tests/helpers/stylesheets.app_css(), which concatenates the stylesheets in
the order index.html loads them. The helper also exposes the <link> markup
for tests that build a synthetic page through Playwright.

tests/test_calendar_visual_static.py

@@ -1,13 +1,25 @@
 from pathlib import Path
 import re
 
+from tests.helpers.stylesheets import app_css
+
 
 ROOT = Path(__file__).resolve().parents[1]
 
 
+def _occurrences(text, needle):
+    """Every index of needle, so an assertion does not depend on which copy of
+    a selector the cascade happens to put first."""
+    out, i = [], text.find(needle)
+    while i != -1:
+        out.append(i)
+        i = text.find(needle, i + 1)
+    return out
+
+
 def test_calendar_week_view_has_overlap_lanes_and_live_ruler_hooks():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert "function _wkLayoutTimedEvents" in calendar_src
     assert "--lane:${lane};--lane-count:${laneCount}" in calendar_src
@@ -20,7 +32,7 @@ def test_calendar_week_view_has_overlap_lanes_and_live_ruler_hooks():
 
 def test_crowded_week_events_expand_left_to_reveal_full_title_on_hover():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert "const crowdedClass = laneCount > 1 ? ' cal-wk-block-crowded' : '';" in calendar_src
     assert "--lane-right:${laneRight}%" in calendar_src
@@ -61,7 +73,7 @@ def test_crowded_week_events_expand_left_to_reveal_full_title_on_hover():
 
 def test_mobile_truncated_week_event_expands_before_opening_editor():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     click_idx = calendar_src.index("body.querySelectorAll('.cal-wk-block, .cal-wk-allday-event')")
     edit_idx = calendar_src.index("if (ev) _showEventForm(ev);", click_idx)
@@ -78,7 +90,7 @@ def test_mobile_truncated_week_event_expands_before_opening_editor():
 
 def test_mobile_empty_week_slot_selects_before_opening_new_event():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert "let _selectedWeekSlot = null;" in calendar_src
     assert "function _paintSelectedWeekSlot(body)" in calendar_src
@@ -100,7 +112,7 @@ def test_mobile_empty_week_slot_selects_before_opening_new_event():
 
 def test_week_view_hints_when_current_time_is_below_the_visible_field():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert "function _updateWeekNowBelowHint(wrap)" in calendar_src
     assert "nowRect.top > wrapRect.bottom - 2" in calendar_src
@@ -115,7 +127,7 @@ def test_week_view_hints_when_current_time_is_below_the_visible_field():
 
 def test_mobile_week_scroll_has_resisted_edge_pull_feedback():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert "pullStartedAtTop = _wrap.scrollTop <= 1;" in calendar_src
     assert "pullStartedAtBottom = _wrap.scrollTop >= maxScroll - 1;" in calendar_src
@@ -143,7 +155,7 @@ def test_calendar_view_change_recovers_a_fully_open_day_drawer():
 
 def test_calendar_month_and_agenda_have_visual_depth_hooks():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert "cal-weekend" in calendar_src
     assert "cal-empty-day" in calendar_src
@@ -156,7 +168,7 @@ def test_calendar_month_and_agenda_have_visual_depth_hooks():
 
 def test_calendar_event_cards_show_compact_source_badges():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert "function _eventSourceHtml" in calendar_src
     assert "${_eventSourceHtml(ev)}" in calendar_src
@@ -166,7 +178,7 @@ def test_calendar_event_cards_show_compact_source_badges():
 
 def test_calendar_toolbar_previous_next_arrows_are_mobile_only():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert 'class="cal-nav cal-toolbar-arrow" id="cal-prev"' in calendar_src
     assert 'class="cal-nav cal-toolbar-arrow" id="cal-next"' in calendar_src
@@ -181,7 +193,7 @@ def test_calendar_toolbar_previous_next_arrows_are_mobile_only():
 
 def test_calendar_side_arrows_center_against_calendar_pane():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert "function _alignSideNavToCalendar()" in calendar_src
     assert "paneRect.top - contentRect.top + (paneRect.height / 2)" in calendar_src
@@ -191,7 +203,7 @@ def test_calendar_side_arrows_center_against_calendar_pane():
 
 def test_calendar_splitter_double_click_snaps_to_nearest_pane_then_toggles():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert "detailH >= calendarH ? 'events' : 'calendar'" in calendar_src
     assert "splitter.addEventListener('dblclick', () => {" in calendar_src
@@ -302,7 +314,7 @@ def test_calendar_settings_are_close_only_and_color_the_name_field():
     assert name_idx < color_idx
     assert "nameInput.style.borderColor = colorInput.value" in calendar_src
     assert "nameInput.style.backgroundColor = colorInput.value" not in calendar_src
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
     toggle_idx = style_src.index(".cal-week-start-toggle")
     button_idx = style_src.index(".cal-week-start-btn", toggle_idx)
     assert "padding: 0;" in style_src[toggle_idx:button_idx]
@@ -312,7 +324,7 @@ def test_calendar_settings_are_close_only_and_color_the_name_field():
 
 def test_calendar_settings_actions_sync_error_and_escape_behavior():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert calendar_src.count('class="cal-settings-actions"') >= 4
     assert "justify-content: flex-end;" in style_src[style_src.index(".cal-settings-actions"):]
@@ -331,7 +343,7 @@ def test_calendar_settings_actions_sync_error_and_escape_behavior():
 
 def test_calendar_hides_navigation_in_agenda_and_event_forms():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert "_modal?.classList.toggle('cal-navigation-hidden', _view === 'agenda')" in calendar_src
     assert "_modal?.classList.add('cal-navigation-hidden')" in calendar_src
@@ -353,7 +365,7 @@ def test_calendar_event_form_escape_cancels_before_modal_close():
 
 
 def test_calendar_from_and_to_month_day_use_theme_highlight():
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     month_idx = style_src.index('input[type="date"]::-webkit-datetime-edit-month-field')
     day_idx = style_src.index('input[type="date"]::-webkit-datetime-edit-day-field', month_idx)
@@ -364,7 +376,7 @@ def test_calendar_from_and_to_month_day_use_theme_highlight():
 
 def test_calendar_location_hint_uses_accent_map_pin():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert 'class="cal-loc-hint"' in calendar_src
     assert '<span>Location</span>' in calendar_src
@@ -382,7 +394,7 @@ def test_calendar_tag_order_prioritizes_personal_work_travel_admin():
 
 def test_long_week_events_keep_their_label_visible_while_scrolling():
     calendar_src = (ROOT / "static/js/calendar.js").read_text()
-    style_src = (ROOT / "static/style.css").read_text()
+    style_src = app_css()
 
     assert 'class="cal-wk-block-label"' in calendar_src
     label_idx = style_src.index(".cal-wk-block-label")
@@ -393,8 +405,12 @@ def test_long_week_events_keep_their_label_visible_while_scrolling():
     block_idx = style_src.index(".cal-wk-block {")
     assert "overflow: clip;" in style_src[block_idx:label_idx]
     head_idx = style_src.index(".cal-wk-col-head {")
-    allday_idx = style_src.rindex(
-        ".cal-wk-allday {", 0, style_src.index(".cal-wk-allday::-webkit-scrollbar")
+    # .cal-wk-allday and its scrollbar rule may now sit in different
+    # stylesheets, so anchoring on the scrollbar rule's offset no longer finds
+    # the block. Take whichever .cal-wk-allday block carries the z-index.
+    allday_idx = next(
+        i for i in _occurrences(style_src, ".cal-wk-allday {")
+        if "z-index: 39;" in style_src[i:i + 500]
     )
     now_idx = style_src.index(".cal-wk-now {")
     assert "z-index: 40;" in style_src[head_idx:head_idx + 500]