Frosted ice glass restyle

Rebuilds the look around the spec: an ice scene, one shared glass
recipe, icy accents and glass on every large surface. Visual only —
layout, behaviour, copy and functionality are untouched.

Background
- .ice-scene: fixed, z-index -1, pointer-events none. Navy base
  (#070b16 to #0c1426) in dark, pale (#f4f9ff to #e6f1fb) in light,
  four drifting glacier blobs (#38bdf8/#60a5fa/#818cf8, 30-60s
  transform-only loops), five glossy ice orbs (40-220px radial
  gradients with a highlight at 30% 25% and a blue shadow) placed so the
  glass blurs them, and an SVG feTurbulence grain at 4-5%.
- prefers-reduced-motion stops the drift; phones get two blobs and two
  orbs fewer.

Glass
- One shared recipe (.glass-panel, .card, .sidebar-surface,
  .study-chrome, .tabbar, .glass, .gate-card): rgba(255,255,255,.06)
  dark / .55 light, blur(24px) saturate(140%) (16px on phones), a bright
  1px rim, an inset top highlight and a blue-tinted outer shadow.
- Nested glass (chips, inputs, secondary buttons) is more opaque and
  never adds a second blur layer. @supports fallback for no
  backdrop-filter. Radii: 24-28px cards/sheets, 14px inputs/buttons,
  pills for chips. Cards hover brighter + cyan rim + 2px lift, 180ms.

Colour and type
- Accent is glacier cyan (#38bdf8) to ice blue (#0ea5e9); progress bars
  and selected states use the gradient; the active sidebar item is a
  frosted highlight with a cyan rim.
- Primary CTA: icy gradient — white to #bfe3ff with ink text in dark,
  deep blue in light (biased so the label sits on the darker half and
  keeps 5.6:1).
- Leftover slate/black greys replaced with navy-tinted frost tokens.
- Headings are semibold with tighter tracking.

Screens
- Dashboard: the sidebar is one tall inset rounded glass panel; deck
  cards are glass.
- Gate: one centred glass card over the scene, orbs visible behind it.
- Sheets/modals: glass over the dimmed, lightly blurred backdrop.
- Study view: question card more opaque (.glass-solid); top bar and
  pager are slim glass pills.

Perf: measured on a phone viewport with 4x CPU throttling. The animated
blobs' filter: blur() was the whole cost (150ms vs 33ms per frame), so
they are soft radial gradients with no filter now; idle 60fps, ~17ms
median while scrolling.

Tests updated for the new tokens: contrast 18/18 AA, deck chrome 15/15,
key gate 49/49, install sheet 64/64, deck fixes 15/15, retry 14/14,
zoom 6/6, final ux 12/12, security 8/8, pwa 16/16.

components/InstallSheet.tsx

@@ -236,7 +236,7 @@ export function InstallSheet() {
               animate={reduceMotion ? { opacity: 1 } : { y: 0 }}
               exit={reduceMotion ? { opacity: 0 } : { y: "100%" }}
               transition={{ duration: 0.34, ease: [0.22, 1, 0.36, 1] }}
-              className="absolute inset-x-0 bottom-0 mx-auto w-full rounded-t-[28px] border border-b-0 border-[var(--install-border)] bg-[var(--install-sheet)] shadow-[0_-20px_60px_-28px_rgb(0_0_0/0.5)] sm:max-w-xl"
+              className="gate-card absolute inset-x-0 bottom-0 mx-auto w-full rounded-b-none border-b-0 sm:max-w-xl"
             >
               <div className="thin-scroll max-h-[90dvh] overflow-y-auto px-5 pt-2 pb-[calc(24px+env(safe-area-inset-bottom))]">
                 <div
@@ -250,7 +250,7 @@ export function InstallSheet() {
                 <div className="flex items-center justify-between gap-4">
                   <h2
                     id="install-sheet-title"
-                    className="text-[17px] font-semibold tracking-[-0.01em] text-[var(--text)]"
+                    className="text-[17px] font-semibold tracking-[-0.02em] text-[var(--text)]"
                   >
                     {info.platform === "ios"
                       ? "Install Freezmo on iPhone"