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/README.md

@@ -153,6 +153,23 @@ The helpers below live under `tests/helpers/`. They exist to remove repeated
 boilerplate that already appeared across multiple tests. Reach for one only when
 your test matches its intended use; do not stretch a helper to cover a new case.
 
+### `tests.helpers.stylesheets.app_css`
+
+Use when a test asserts on a CSS rule.
+
+- Returns every app stylesheet concatenated in the order `static/index.html`
+  loads them, which is the order the cascade actually has.
+- Panel styles no longer all live in `static/style.css`; reading that file
+  alone ties the test to whichever file a rule sits in today, so it goes red
+  when a rule moves without the rendered page changing.
+- `stylesheet_paths()` and `stylesheet_urls()` are there when a test needs the
+  files or the request URLs rather than their contents.
+  `stylesheet_link_tags()` returns the `<link>` markup for a synthetic page
+  driven through Playwright, so it gets the whole cascade instead of only
+  `style.css`.
+- All of them fail loudly if `index.html` links a stylesheet that is missing.
+- Not for vendored CSS under `static/lib/`, which they deliberately skip.
+
 ### `tests.helpers.cli_loader.load_script`
 
 Use when a test needs to import a script under `scripts/` without repeating