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.

components/chat/ModelPicker.tsx

@@ -122,7 +122,7 @@ export function ModelPicker({
               animate={reduceMotion ? { opacity: 1 } : { y: 0 }}
               exit={reduceMotion ? { opacity: 0 } : { y: "100%" }}
               transition={reduceMotion ? { duration: 0 } : springSheet}
-              className="glass-panel absolute inset-x-0 bottom-0 max-h-[80dvh] rounded-b-none px-4 pb-safe pt-2"
+              className="glass-panel glass-sheet absolute inset-x-0 bottom-0 max-h-[80dvh] rounded-b-none px-4 pb-safe pt-2"
             >
               <div className="sheet-grabber mx-auto" aria-hidden />
               <h2 className="px-1 pb-3 pt-2 text-[15px] font-semibold text-[var(--text)]">{t("chat.modelTitle")}</h2>