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.

lib/i18n-dict.ts

@@ -1001,6 +1001,9 @@ const en: Record<string, string> = {
   "legal.privacy": "Privacy policy",
   "legal.terms": "Terms of service",
   "legal.cookies": "Cookie policy",
+  "settings.theme": "Theme",
+  "theme.aero": "Frutiger Aero",
+  "theme.aeroNote": "Aero has no light/dark switch — pick Light or Dark to get it back.",
   "settings.legal": "Legal",
   "consent.title": "About cookies",
   "consent.body": "Freezmo uses no cookies and no trackers — your decks, homework and settings stay in this browser.",
@@ -1953,6 +1956,9 @@ const cs: Record<string, string> = {
   "legal.privacy": "Zásady soukromí",
   "legal.terms": "Podmínky použití",
   "legal.cookies": "Zásady cookies",
+  "settings.theme": "Téma",
+  "theme.aero": "Frutiger Aero",
+  "theme.aeroNote": "Aero nemá přepínač světlý/tmavý — vyber Světlý nebo Tmavý, pokud ho chceš zpět.",
   "settings.legal": "Právní informace",
   "consent.title": "O cookies",
   "consent.body": "Freezmo nepoužívá cookies ani sledování — sady, úkoly a nastavení zůstávají v tomto prohlížeči.",