refactor(tests): read the whole cascade instead of style.css alone

static/style.css no longer holds every rule. 79 rules for the document,
gallery and editor panels now live in static/css/, yet 27 browser tests still
built their synthetic page with a single <link> to style.css, and 51 more read
that one file as though it were the whole cascade. Those tests kept passing
while covering less: a rule that moved became invisible to the assertion that
was meant to pin it.

Python readers now call tests.helpers.stylesheets.app_css(), and synthetic
pages are built from stylesheet_link_tags() so they load exactly what
index.html loads, in the same order. The helper already existed; this moves
the remaining callers onto it.

test_portal_dropdown_z_js parametrised over a file list including style.css to
assert an absence. Checking a negative against one file of a split stylesheet
is how a moved rule escapes, so the CSS case now checks the concatenation.

Two guards keep it from coming back: one fails on any test reading
static/style.css directly, the other on any synthetic page linking it alone.
Both name the helper to use.

No production code changes. Suite is unchanged at 6 pre-existing failures.

tests/test_document_editor_scroll.py

@@ -9,11 +9,12 @@ document.js is browser-coupled and not importable in pytest.
 """
 
 from pathlib import Path
+from tests.helpers.stylesheets import app_css
 
 
 ROOT = Path(__file__).resolve().parents[1]
 DOC_JS = (ROOT / "static/js/document.js").read_text(encoding="utf-8")
-STYLE_CSS = (ROOT / "static/style.css").read_text(encoding="utf-8")
+STYLE_CSS = app_css()
 
 
 def test_document_textarea_scrollbar_is_visible():