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_action_menu_order.py

@@ -1,6 +1,8 @@
 from pathlib import Path
 import subprocess
 
+from tests.helpers.stylesheets import app_css
+
 
 ROOT = Path(__file__).resolve().parents[1]
 
@@ -80,7 +82,7 @@ def test_email_filter_menu_has_context_title() -> None:
 
 def test_email_setting_toggles_render_neutral_disabled_state() -> None:
     source = (ROOT / "static/js/emailLibrary.js").read_text(encoding="utf-8")
-    style = (ROOT / "static/style.css").read_text(encoding="utf-8")
+    style = app_css()
     assert 'email-settings-auto-reply-section' in source
     assert 'email-settings-display-enabled-state' in source
     assert 'stateLabel = section?.querySelector' in source
@@ -100,7 +102,7 @@ def test_email_date_headers_mark_unexpected_timeline_gaps() -> None:
     assert "email-date-gap-break" in source
     assert "gapDays > 90 && gapDays > timelineGapThreshold" in source
 
-    style = (ROOT / "static/style.css").read_text(encoding="utf-8")
+    style = app_css()
     assert ".date-section-header.email-date-gap-break" in style
 
 
@@ -122,7 +124,7 @@ def test_email_filters_and_card_favorite_toggle_are_wired() -> None:
     assert "const typedFilter = _exactTypedFilterSuggestion(v);" in source
     assert "_acceptSuggestion(typedFilter);" in source
 
-    style = (ROOT / "static/style.css").read_text(encoding="utf-8")
+    style = app_css()
     favorite_start = style.index(".email-card-favorite {")
     favorite_end = style.index("}", favorite_start) + 1
     assert "top: -3px;" in style[favorite_start:favorite_end]
@@ -154,7 +156,7 @@ def test_email_auto_reply_syncs_one_calendar_event_per_account() -> None:
 
 def test_email_settings_show_away_account_and_compact_display_controls() -> None:
     source = (ROOT / "static/js/emailLibrary.js").read_text(encoding="utf-8")
-    style = (ROOT / "static/style.css").read_text(encoding="utf-8")
+    style = app_css()
     assert 'email-account-away-label">(AWAY)</span>' in source
     assert 'id="email-lib-auto-reply-badge"' in source
     assert ">Show Email Tags</span>" in source
@@ -227,7 +229,7 @@ def test_email_clean_always_forces_a_fresh_unsubscribe_scan() -> None:
 
 def test_unsubscribe_duplicate_badge_is_lowered() -> None:
     frontend = (ROOT / "static/js/emailLibrary.js").read_text(encoding="utf-8")
-    stylesheet = (ROOT / "static/style.css").read_text(encoding="utf-8")
+    stylesheet = app_css()
     assert "email-unsub-duplicate-badge" in frontend
     start = stylesheet.index(".email-unsub-duplicate-badge {")
     assert "top: 2px;" in stylesheet[start:stylesheet.index("}", start) + 1]
@@ -235,7 +237,7 @@ def test_unsubscribe_duplicate_badge_is_lowered() -> None:
 
 def test_unsubscribe_scan_status_sits_before_clean_action() -> None:
     frontend = (ROOT / "static/js/emailLibrary.js").read_text(encoding="utf-8")
-    stylesheet = (ROOT / "static/style.css").read_text(encoding="utf-8")
+    stylesheet = app_css()
     start = frontend.index("function _emailCleanupSettingsHtml")
     end = frontend.index("function _emailDisplaySettingsHtml", start)
     cleanup = frontend[start:end]
@@ -291,7 +293,7 @@ def test_browser_agent_unsubscribe_cleans_sender_after_positive_confirmation() -
 
 
 def test_auto_unsubscribe_all_is_visibly_taller_than_toolbar_buttons() -> None:
-    source = (ROOT / "static/style.css").read_text(encoding="utf-8")
+    source = app_css()
     start = source.index(".email-unsub-auto-safe-btn {")
     assert "height: 29px;" in source[start:source.index("}", start) + 1]
 
@@ -330,7 +332,7 @@ def test_unsubscribe_review_marks_handled_cards_and_offers_scan_further() -> Non
 
 def test_unsubscribe_review_can_ignore_a_candidate_without_deleting_it() -> None:
     source = (ROOT / "static" / "js" / "emailLibrary.js").read_text()
-    styles = (ROOT / "static" / "style.css").read_text()
+    styles = app_css()
     assert "email-unsub-ignore-btn" in source
     assert "_rememberUnsubscribeIgnored(c)" in source
     assert "Ignore this unsubscribe candidate" in source
@@ -339,7 +341,7 @@ def test_unsubscribe_review_can_ignore_a_candidate_without_deleting_it() -> None
 
 def test_email_settings_sections_use_static_headers() -> None:
     source = (ROOT / "static" / "js" / "emailLibrary.js").read_text()
-    styles = (ROOT / "static" / "style.css").read_text()
+    styles = app_css()
     assert 'class="email-unsub-accent-icon"' in source
     assert 'M12 0L14.59 8.41' in source
     assert 'Scanning ${_esc(scanFolderLabel)} headers…' in source