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" && (

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">

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")}