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/contrast.mjs

@@ -48,7 +48,7 @@ const screens = [
 
 for (const [label, path, mode] of screens) {
   if (filter && !label.includes(filter)) continue;
-  for (const theme of ["dark", "light"]) {
+  for (const theme of ["dark", "light", "aero"]) {
     const { context, page } = await makePage(browser, { width: 390, height: 844, deviceScaleFactor: 1 });
     await seed(page, { decks: [DECK], theme });
     await page.goto(`${BASE}${path}`, { waitUntil: "networkidle" });