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/DeckCard.tsx

@@ -114,7 +114,7 @@ export function DeckCard({ deck, onMove }: { deck: Deck; onMove?: (folderId: str
         href={`/deck/${deck.id}`}
         aria-hidden="true"
         tabIndex={-1}
-        className="absolute inset-0 hidden rounded-[1.125rem] lg:block"
+        className="absolute inset-0 hidden rounded-[1.5rem] lg:block"
       />
 
       {/* header ------------------------------------------------------- */}
@@ -322,7 +322,7 @@ export function DeckCard({ deck, onMove }: { deck: Deck; onMove?: (folderId: str
                     className={cn(
                       "rounded-full px-2.5 py-1 text-[12px] font-medium transition",
                       current
-                        ? "bg-[var(--text)] text-white dark:bg-white dark:text-[#0d0d0d]"
+                        ? "grad-ice text-[#06111f]"
                         : "text-[var(--text-muted)] hover:bg-[var(--hover)] hover:text-[var(--text)]",
                     )}
                   >
@@ -399,7 +399,7 @@ export function DeckCard({ deck, onMove }: { deck: Deck; onMove?: (folderId: str
                   className={cn(
                     "min-w-9 rounded-full px-2.5 py-1 text-[12px] font-medium transition",
                     addCount === option
-                      ? "bg-[var(--text)] text-white dark:bg-white dark:text-[#0d0d0d]"
+                      ? "grad-ice text-[#06111f]"
                       : "text-[var(--text-muted)] hover:bg-[var(--hover)] hover:text-[var(--text)]",
                   )}
                 >