fix(ui): opacity-based frosted glass; drop full-page settings blur

Evidence (sharp video behind overlays) shows backdrop-filter is a no-op here, most
likely because browser hardware acceleration is off — so the glass relied on a blur
that never rendered. Lean on opacity instead (94% surface) so overlay menus/panels
read as frosted glass regardless of GPU; keep the blur as a bonus when available.
Revert the settings backdrop to a plain dim scrim (no whole-page blur) and make the
header a solid bar (no backdrop-filter ancestor).

frontend/src/components/Header.tsx

@@ -26,7 +26,7 @@ export default function Header({
   }
 
   return (
-    <header className="h-14 shrink-0 border-b border-border bg-surface/80 backdrop-blur flex items-center gap-3 px-4 z-20">
+    <header className="h-14 shrink-0 border-b border-border bg-surface/95 flex items-center gap-3 px-4 z-20">
       <button
         onClick={() => setPage("feed")}
         className="text-xl font-bold tracking-tight select-none"

frontend/src/components/SettingsPanel.tsx

@@ -54,7 +54,7 @@ export default function SettingsPanel({
   return (
     <div className="fixed inset-0 z-40 flex justify-end">
       <div
-        className={`absolute inset-0 bg-black/30 backdrop-blur-lg ${
+        className={`absolute inset-0 bg-black/50 ${
           closing ? "animate-[overlayOut_0.19s_ease_forwards]" : "animate-[overlayIn_0.2s_ease]"
         }`}
         onClick={close}