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/Sidebar.tsx

@@ -31,7 +31,7 @@ function TagChip({
     <button
       onClick={onClick}
       title={`${tag.channel_count} channel${tag.channel_count === 1 ? "" : "s"}`}
-      className={`text-xs px-2.5 py-1 rounded-full border transition ${
+      className={`text-xs px-2.5 py-1 rounded-full border shadow-sm hover:shadow active:translate-y-px transition ${
         active
           ? "bg-accent text-accent-fg border-accent"
           : "bg-card border-border text-fg hover:border-accent"
@@ -77,7 +77,7 @@ export default function Sidebar({
             <button
               key={s.id}
               onClick={() => setFilters({ ...filters, show: s.id })}
-              className={`text-xs py-1.5 rounded-lg border transition ${
+              className={`text-xs py-1.5 rounded-lg border shadow-sm active:translate-y-px transition ${
                 filters.show === s.id
                   ? "bg-accent text-accent-fg border-accent"
                   : "bg-card border-border hover:border-accent"