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_cookbook_dependency_completion_regression.py

@@ -1,5 +1,7 @@
 from pathlib import Path
 
+from tests.helpers.stylesheets import app_css
+
 
 ROOT = Path(__file__).resolve().parents[1]
 
@@ -302,7 +304,7 @@ def test_dependency_status_cache_is_not_fragmented_by_unused_model_hint():
 def test_model_lists_fade_after_complete_rows_are_painted():
     hwfit = _read("static/js/cookbook-hwfit.js")
     cookbook = _read("static/js/cookbook.js")
-    style = _read("static/style.css")
+    style = app_css()
 
     assert "cookbook-model-list-fade" in hwfit
     assert "cookbook-model-list-fade" in cookbook
@@ -314,7 +316,7 @@ def test_model_lists_fade_after_complete_rows_are_painted():
 
 def test_dependency_panel_has_stable_categories_and_mobile_build_action():
     cookbook = _read("static/js/cookbook.js")
-    style = _read("static/style.css")
+    style = app_css()
 
     assert "const _depCategoryOrder = ['System', 'Tools', 'LLM', 'Image'" in cookbook
     assert "const _orderedDepCategories = (byCat)" in cookbook
@@ -337,7 +339,7 @@ def test_direct_download_auto_fold_requires_deliberate_boundary_gesture():
 
 
 def test_launch_command_box_uses_accent_focus_glow():
-    style = _read("static/style.css")
+    style = app_css()
     assert ".hwfit-serve-cmd-details[open] .hwfit-serve-cmd" in style
     assert "box-shadow: 0 0 0 1px color-mix(in srgb, var(--accent" in style
 
@@ -345,7 +347,7 @@ def test_launch_command_box_uses_accent_focus_glow():
 def test_server_settings_color_picker_uses_dot_trigger():
     cookbook = _read("static/js/cookbook.js")
     hwfit = _read("static/js/cookbook-hwfit.js")
-    style = _read("static/style.css")
+    style = app_css()
     assert 'aria-label="Change server color (currently ${esc(selectedColorLabel)})"' in cookbook
     assert "btn.setAttribute('aria-label', `Change server color (currently ${label})`)" in hwfit
     assert ".cookbook-server-row .cookbook-srv-color-dot" in style
@@ -357,7 +359,7 @@ def test_server_settings_color_picker_uses_dot_trigger():
 def test_default_server_matches_exclusive_model_directory_selector():
     cookbook = _read("static/js/cookbook.js")
     hwfit = _read("static/js/cookbook-hwfit.js")
-    style = _read("static/style.css")
+    style = app_css()
     assert "const icon = active ? _MODELDIR_CHECK_ON : _MODELDIR_CHECK_OFF;" in cookbook
     assert "_envState.defaultServer = key;" in hwfit
     assert "Toggle off if it's already the default" not in hwfit