Frutiger Aero theme, pixel-measured contrast fixes, sheet surfaces

Frutiger Aero (Settings -> Theme): a third theme, not a dark/light variant.
A sky gradient with a strip of grass, the glacier blobs become clouds, the
orbs soap bubbles, glass gets the classic horizon gloss and the buttons a
glossy aqua. The light/dark toggles (header, rail, chat header) hide while it
is on and the picker says why; the pre-paint script applies the class before
first paint and the PWA status bar follows (#3fa3e0). Its muted/frost text is
darker than Light's because those labels sit directly on the sky.

Contrast, measured on real pixels (tests/contrast.mjs, now covering aero):
- The light settings sheet was translucent over a dark scrim, so its muted
  text measured ~2.8:1. Sheets and dialogs now use --glass-sheet-bg (a more
  opaque surface), and Light's --text-muted is a touch darker.
- Result: 0 below-minimum findings and 0 axe violations across every screen,
  all three themes, phone and desktop.

Also: the suites now seed the cookie notice as already dismissed (they run as
a returning user); the regression suite opts out to test the notice itself —
the previous deploy failed its gate because the notice blocked a chat chip
click. The notice also hides during immersive sessions (overlay-open), so it
can never cover a study feed.

Version 0.1.8, SW -> freezmo-v54.

tests/lib.mjs

@@ -96,15 +96,19 @@ export async function seed(
     key = "test-key",
     extra = {},
     onlyIfMissing = false,
+    /** Suites run as a returning user: the cookie notice is already dismissed.
+     *  The regression suite passes false to test the notice itself. */
+    consent = true,
   } = {},
 ) {
   await page.addInitScript(
-    ({ locale, theme, decks, folders, homework, key, extra, onlyIfMissing }) => {
+    ({ locale, theme, decks, folders, homework, key, extra, onlyIfMissing, consent }) => {
       const put = (name, value) => {
         if (onlyIfMissing && localStorage.getItem(name) !== null) return;
         localStorage.setItem(name, value);
       };
       put("freezmo.install.installed", "1");
+      if (consent) put("freezmo.consent.v1", JSON.stringify({ at: 1 }));
       put("freezmo.locale.v1", locale);
       put("freezmo.theme", theme);
       if (key) put("freezmo.settings.v1", JSON.stringify({ apiKey: key }));
@@ -113,7 +117,7 @@ export async function seed(
       put("freezmo.homework.v1", JSON.stringify(homework));
       for (const [name, value] of Object.entries(extra)) put(name, value);
     },
-    { locale, theme, decks, folders, homework, key, extra, onlyIfMissing },
+    { locale, theme, decks, folders, homework, key, extra, onlyIfMissing, consent },
   );
 }