fix: feedback round 2 — language, subscriptions, feed scope, UX

- Language detection: classify one cleaned+concatenated blob (strip emoji,
  @mentions, #tags, numbers, punctuation); fixes caps/emoji-heavy channels
  (e.g. Nessaj -> Hungarian, no more bogus Chinese/Korean)
- Feed now joins the user's subscriptions, so unsubscribing on YouTube removes a
  channel from the feed; periodic subscription re-sync job picks up changes
- Watched/Saved/Hidden views ignore the Shorts/live default-hiding so the full
  set is visible (fixes hidden videos missing from the Hidden view)
- Persist feed filters + search across reloads (localStorage)
- 3D polish: cards lift with shadow on hover; chips/buttons get depth and a
  press effect; undo toast lasts longer

frontend/src/components/VideoCard.tsx

@@ -56,7 +56,7 @@ function Thumb({ video, className }: { video: Video; className?: string }) {
       rel="noreferrer"
       onClick={() => video.status === "new" && undefined}
       className={clsx(
-        "block relative rounded-xl overflow-hidden bg-surface border border-border",
+        "block relative rounded-xl overflow-hidden bg-surface border border-border shadow-md group-hover:shadow-2xl transition-shadow",
         className
       )}
     >
@@ -120,7 +120,7 @@ export default function VideoCard({
     return (
       <div
         className={clsx(
-          "group flex gap-3 p-2 rounded-xl hover:bg-card transition",
+          "group flex gap-3 p-2 rounded-xl hover:bg-card hover:shadow-lg transition",
           watched && "opacity-55"
         )}
       >
@@ -144,7 +144,12 @@ export default function VideoCard({
   }
 
   return (
-    <div className={clsx("group", watched && "opacity-55")}>
+    <div
+      className={clsx(
+        "group transition-transform duration-150 hover:-translate-y-1",
+        watched && "opacity-55"
+      )}
+    >
       <Thumb video={video} className="aspect-video" />
       <div className="flex gap-3 mt-2">
         {video.channel_thumbnail && (