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/ApiKeyGate.tsx

@@ -181,7 +181,7 @@ function GateScreen() {
   return (
     <div
       ref={rootRef}
-      className="gate-bg fixed inset-0 h-dvh overflow-hidden overscroll-none"
+      className="fixed inset-0 h-dvh overflow-hidden overscroll-none"
     >
       {/* corner controls: theme + help ---------------------------------- */}
       <div
@@ -230,7 +230,7 @@ function GateScreen() {
           initial={reduceMotion ? false : { opacity: 0, y: 14 }}
           animate={{ opacity: 1, y: 0 }}
           transition={{ duration: 0.45, ease: [0.22, 1, 0.36, 1] }}
-          className="my-auto w-full max-w-[420px] rounded-[24px] border border-[var(--gate-card-border)] bg-[var(--gate-card)] p-6 px-7 text-center shadow-[var(--shadow-gate)] sm:p-8"
+          className="gate-card my-auto w-full max-w-[420px] p-6 px-7 text-center sm:p-8"
         >
           {/* eslint-disable-next-line @next/next/no-img-element -- static app icon, no optimisation needed */}
           <img
@@ -240,7 +240,7 @@ function GateScreen() {
             className="mx-auto size-14 rounded-[16px] shadow-[var(--shadow-card)]"
           />
 
-          <h1 className="mt-6 text-[24px] font-medium leading-tight tracking-[-0.01em] text-[var(--text)] sm:text-[28px]">
+          <h1 className="mt-6 text-[24px] font-semibold leading-tight tracking-[-0.02em] text-[var(--text)] sm:text-[28px]">
             Connect your Ollama key
           </h1>
           <p className="mx-auto mt-1.5 max-w-[300px] text-balance text-[15px] leading-relaxed text-[var(--text-subtle)]">
@@ -363,12 +363,12 @@ function HelpDialog({ open, onClose }: { open: boolean; onClose: () => void }) {
             exit={reduceMotion ? { opacity: 0 } : { y: 24, opacity: 0 }}
             transition={{ type: "spring", stiffness: 340, damping: 32 }}
             onClick={(event) => event.stopPropagation()}
-            className="thin-scroll max-h-[90dvh] w-full max-w-[480px] overflow-y-auto rounded-t-[24px] border border-[var(--gate-card-border)] bg-[var(--gate-card)] p-6 pb-safe shadow-[var(--shadow-gate)] sm:rounded-[24px] sm:pb-6"
+            className="gate-card thin-scroll max-h-[90dvh] w-full max-w-[480px] overflow-y-auto p-6 pb-safe sm:pb-6"
           >
             <div className="flex items-center justify-between gap-4">
               <h2
                 id="gate-help-title"
-                className="text-[19px] font-medium leading-snug tracking-[-0.01em] text-[var(--text)]"
+                className="text-[19px] font-semibold leading-snug tracking-[-0.02em] text-[var(--text)]"
               >
                 How to get your Ollama API key
               </h2>