feat(ui): glassmorphism phase 1 — frost the app chrome

Apply the real frosted .glass surface (blur) to the few large chrome surfaces — the nav
sidebar, the header and all Modal-based dialogs (was glass-card / plain surfaces) — and
enrich the ambient backdrop a touch (three soft accent pools) so the glass has more to
refract app-wide. Popovers (notifications, language, account, add-to-playlist) were
already glass. Bulk feed cards stay glass-card (no per-card blur) for performance; the
existing perf-mode still disables blur. Phase 2 (ambient thumbnail mosaic / bg image +
toggle) deferred to end-of-project polish.

frontend/src/components/Modal.tsx

@@ -35,7 +35,7 @@ export default function Modal({
       aria-modal="true"
     >
       <div
-        className={`glass-card relative w-full ${maxWidth} max-h-full overflow-y-auto rounded-2xl shadow-2xl`}
+        className={`glass relative w-full ${maxWidth} max-h-full overflow-y-auto rounded-2xl shadow-2xl`}
         onClick={(e) => e.stopPropagation()}
       >
         <div className="flex items-start justify-between gap-3 p-5 pb-3">