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/Sidebar.tsx

@@ -362,11 +362,14 @@ export function Sidebar() {
     <aside
       aria-label="Sidebar"
       className={cn(
-        "sidebar-surface fixed inset-y-0 left-0 z-40 hidden flex-col px-2.5 py-3 transition-[width] duration-200 lg:flex",
-        isCollapsed ? "w-[68px]" : "w-[260px]",
+        "fixed inset-y-0 left-0 z-40 hidden p-3 transition-[width] duration-200 lg:flex",
+        isCollapsed ? "w-[84px]" : "w-[268px]",
       )}
     >
-      <NavContent collapsed={isCollapsed} desktop />
+      {/* One tall inset glass panel, so the ice scene shows around it. */}
+      <div className="sidebar-surface flex min-h-0 flex-1 flex-col px-2.5 py-3">
+        <NavContent collapsed={isCollapsed} desktop />
+      </div>
     </aside>
   );
 }
@@ -395,7 +398,7 @@ export function MobileSidebar() {
             animate={{ x: 0 }}
             exit={{ x: -280 }}
             transition={{ type: "spring", stiffness: 380, damping: 36 }}
-            className="sidebar-surface relative z-10 flex h-full w-[272px] flex-col px-2.5 py-3 pb-safe"
+            className="sidebar-surface relative z-10 flex h-full w-[272px] flex-col rounded-l-none rounded-r-[1.5rem] px-2.5 py-3 pb-safe"
           >
             <NavContent onNavigate={() => sidebarOpen.set(false)} />
           </motion.aside>
@@ -411,7 +414,7 @@ export function SidebarOffset() {
   return (
     <div
       aria-hidden
-      className={cn("hidden shrink-0 transition-[width] duration-200 lg:block", isCollapsed ? "w-[68px]" : "w-[260px]")}
+      className={cn("hidden shrink-0 transition-[width] duration-200 lg:block", isCollapsed ? "w-[84px]" : "w-[268px]")}
     />
   );
 }