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/Header.tsx

@@ -28,7 +28,7 @@ export default function Header({
   const { t, i18n } = useTranslation();
 
   return (
-    <header className="h-14 shrink-0 border-b border-border bg-surface/95 flex items-center gap-3 px-4 z-20">
+    <header className="glass h-14 shrink-0 border-b border-border flex items-center gap-3 px-4 z-20">
       <SyncStatus isAdmin={me.role === "admin"} onGoToFullHistory={onGoToFullHistory} />
 
       {page === "feed" && (