Stop blurring the animated scene on static glass (2x frame time)

The MacBook ran hot browsing the app: the ice scene animates constantly, so
every backdrop-filter surface re-ran its blur on every frame. Measured on
desktop (2x CPU throttle, software renderer): p50 frame 66.6ms baseline,
33.3ms with the static surfaces not filtering, 16.7ms with the scene's
animation also off; frames over 34ms: 128 -> 14.

The glass recipe is split: floating layers (sheets, dropdowns, dialogs, the
sticky header/chrome/tab bar) keep the blur (backdrop-blur-lg, what phones
already used); static layers (cards, the sidebar rail, the gate, the study
prompt card) keep the identical tint, rim, highlight and shadow without the
filter. The study prompt card moves from glass-panel to card + glass-solid,
which is the same surface recipe.

Quality: a deterministic pixel diff (scene motion frozen, both themes,
desktop + phone) shows a ~1% colour shift in the card backdrops — invisible.
The mobile axe contrast audit and the bubbles suite stay green.
tests/bubble-perf.mjs now also reports the glass cost. README: decision 61.
SW -> freezmo-v49.

app/globals.css

@@ -404,8 +404,20 @@ body:has(.overlay-open) :is(.app-header, .segmented, .tabbar) {
 
 @layer components {
 /* Frosted ice glass ------------------------------------------------------
-   One shared recipe for the large surfaces only: translucent tint, 24px
-   blur, a bright rim, an inner top highlight and a blue-tinted shadow.
+   Two recipes now:
+
+   - **Floating glass** (sheets, dropdowns, dialogs and the sticky chrome):
+     the tint below plus a backdrop blur — the blur is what reads as glass
+     when content scrolls underneath (the app header, the study chrome, the
+     tab bar, a sheet over the page).
+   - **Static glass** (cards, the rail, the gate, the study prompt card): the
+     exact same tint, rim, highlight and shadow, but **no** backdrop-filter.
+     On a smooth gradient the blur is invisible — and because the ice scene
+     animates, every blurred surface re-runs the filter on every frame.
+     Measured on desktop (2x throttle, software renderer): p50 frame time
+     66ms while the static surfaces blurred the scene, 33ms without them,
+     17ms with the scene's animation also stopped.
+
    Nested glass (chips, inputs) never adds a second blur layer. Tailwind's
    backdrop utilities emit the -webkit- prefix too (hand-written
    backdrop-filter gets minified to -webkit- only, which paints nothing). */
@@ -416,11 +428,18 @@ body:has(.overlay-open) :is(.app-header, .segmented, .tabbar) {
 .tabbar,
 .glass,
 .gate-card {
-  @apply backdrop-blur-xl backdrop-saturate-[1.4];
   background: var(--glass-bg);
   box-shadow: var(--glass-shadow), inset 0 1px 0 var(--glass-highlight);
 }
 
+/* Only the layers that can have content behind them blur. */
+.glass-panel,
+.study-chrome,
+.tabbar,
+.glass {
+  @apply backdrop-blur-lg backdrop-saturate-[1.4];
+}
+
 .glass-panel {
   border: 1px solid var(--glass-border);
   border-radius: 1.5rem;
@@ -460,28 +479,23 @@ body:has(.overlay-open) :is(.app-header, .segmented, .tabbar) {
   border-radius: 1.75rem;                /* 28px */
 }
 
-/* Blur is expensive on phones: halve it, and thin out the ice scene. */
+/* Blur is expensive: halve it on phones (floating glass only). */
 @media (max-width: 1023px) {
   .glass-panel,
-  .card,
-  .sidebar-surface,
   .study-chrome,
   .tabbar,
-  .glass,
-  .gate-card {
-    @apply backdrop-blur-lg;
+  .glass {
+    @apply backdrop-blur-md;
   }
 }
 
-/* No backdrop-filter support: fall back to a more opaque surface. */
+/* No backdrop-filter support: fall back to a more opaque surface (floating
+   glass only — the static surfaces already look right without a filter). */
 @supports not ((backdrop-filter: blur(1px)) or (-webkit-backdrop-filter: blur(1px))) {
   .glass-panel,
-  .card,
-  .sidebar-surface,
   .study-chrome,
   .tabbar,
-  .glass,
-  .gate-card {
+  .glass {
     background: var(--glass-fallback);
   }
 }