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_gallery_visual_static.py

@@ -1,6 +1,8 @@
 from pathlib import Path
 import re
 
+from tests.helpers.stylesheets import app_css
+
 
 ROOT = Path(__file__).resolve().parent.parent
 
@@ -20,7 +22,7 @@ def test_gallery_exposes_active_filter_organization():
 
 def test_gallery_cards_keep_badges_without_hover_inspect_label():
     js = _read("static/js/gallery.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert "gallery-card-overlay" in js
     assert "gallery-card-inspect" not in js
@@ -31,7 +33,7 @@ def test_gallery_cards_keep_badges_without_hover_inspect_label():
 
 def test_gallery_detail_has_richer_inspector_actions():
     js = _read("static/js/gallery.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert "gallery-detail-inspector-head" in js
     assert "gallery-detail-meta-grid" in js
@@ -57,7 +59,7 @@ def test_gallery_static_cache_key_bumped():
 
 def test_gallery_bulk_select_button_keeps_toolbar_alignment():
     js = _read("static/js/gallery.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert 'id="gallery-select-btn"' in js
     assert 'id="gallery-select-btn" title="Select for bulk actions" style="position:relative;top:0;"' in js
@@ -74,7 +76,7 @@ def test_gallery_bulk_select_button_keeps_toolbar_alignment():
 def test_gallery_selection_bars_share_style_and_escape_priority():
     gallery = _read("static/js/gallery.js")
     ui = _read("static/js/ui.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert gallery.count("memory-bulk-bar gallery-selection-bar hidden") == 3
     assert "window.__galleryCancelSelection = () =>" in gallery
@@ -93,7 +95,7 @@ def test_gallery_selection_bars_share_style_and_escape_priority():
 
 def test_editor_saved_projects_support_grid_and_list_views():
     gallery = _read("static/js/gallery.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert 'id="gallery-editor-drafts-view" role="group"' in gallery
     assert 'data-view="grid"' in gallery
@@ -136,7 +138,7 @@ def test_editor_saved_projects_support_grid_and_list_views():
 
 def test_empty_albums_show_action_tiles_instead_of_no_albums_message():
     gallery = _read("static/js/gallery.js")
-    css = _read("static/style.css")
+    css = app_css()
 
     assert "function _albumActionTiles()" in gallery
     assert 'id="gallery-albums-new"' in gallery
@@ -149,7 +151,7 @@ def test_empty_albums_show_action_tiles_instead_of_no_albums_message():
 
 
 def test_gallery_body_uses_available_window_height_without_grid_crop():
-    css = _read("static/style.css")
+    css = app_css()
 
     gallery_section = css.index("/* ── Gallery (image library) ── */")
     modal_start = css.index(".gallery-modal-content {", gallery_section)
@@ -161,7 +163,7 @@ def test_gallery_body_uses_available_window_height_without_grid_crop():
 
 
 def test_gallery_photo_search_is_not_vertically_offset():
-    css = _read("static/style.css")
+    css = app_css()
 
     search_idx = css.index(".gallery-search-wrap {")
     assert "top: 0;" in css[search_idx:search_idx + 180]