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.

components/activities/shared.tsx

@@ -78,7 +78,7 @@ export function PromptCard({
 }) {
   return (
     <div
-      className={cn("glass-panel glass-solid rounded-[1.5rem] px-5 py-6 text-balance sm:px-6", className)}
+      className={cn("card glass-solid rounded-[1.5rem] px-5 py-6 text-balance sm:px-6", className)}
       style={style}
     >
       {children}