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_settings_shell_js_behavior.py

@@ -14,6 +14,7 @@ import subprocess
 from pathlib import Path
 
 import pytest
+from tests.helpers.stylesheets import app_css
 
 
 _REPO = Path(__file__).resolve().parent.parent
@@ -22,11 +23,9 @@ _COORDINATOR_HELPER = (
     _REPO / "tests" / "helpers" / "test_settings_shell_coordinator.mjs"
 )
 _HAS_NODE = shutil.which("node") is not None
-_STYLE = _REPO / "static" / "style.css"
-
-
+_STYLE_TEXT = app_css()
 def test_settings_desktop_width_targets_settings_not_cookbook():
-    source = _STYLE.read_text(encoding="utf-8")
+    source = _STYLE_TEXT
 
     settings_rule = re.search(
         r"(?ms)^\.settings-modal-content\s*\{[^}]*"