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/NavSidebar.tsx

@@ -63,7 +63,7 @@ export default function NavSidebar({
 
   return (
     <nav
-      className={`relative shrink-0 border-r border-border bg-surface/60 flex flex-col py-3 transition-[width] ${
+      className={`glass relative shrink-0 border-r border-border flex flex-col py-3 transition-[width] ${
         collapsed ? "w-[56px] px-2" : "w-52 px-3"
       }`}
       aria-label={t("nav.primary")}