feat(tags): tag UX overhaul + per-card video reset

- Channel-row tags show only what's attached; a '+' opens a per-channel picker. Clicking a tag
  filters the feed by it; a 'Your tags' sidebar widget makes that filter visible/clearable.
- Tag manager dialog (add/rename/delete; delete only confirms when the tag is in use), reachable
  from the Channel manager and the feed sidebar; hovering a tag's count lists its channels, each a
  link that focuses it in the Channel manager (DataTable gains an external filter input).
- Video cards get a reset action that clears all watch state (incl. an in-progress position).
- api.req() now raises the error dialog on 5xx and 400/409/422 with the server's reason.

backend/app/routes/feed.py

@@ -435,6 +435,29 @@ def set_video_state(
     return {"video_id": video_id, "status": status}
 
 
+@router.delete("/videos/{video_id}/state")
+def clear_video_state(
+    video_id: str,
+    user: User = Depends(current_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    """Reset a video to pristine for this user — drop the whole VideoState row (status,
+    watch position and watched_at), as if it had never been opened. Saved/playlist membership
+    lives elsewhere and is untouched."""
+    row = db.execute(
+        select(VideoState).where(
+            VideoState.user_id == user.id, VideoState.video_id == video_id
+        )
+    ).scalar_one_or_none()
+    if row is not None:
+        db.delete(row)
+        try:
+            db.commit()
+        except StaleDataError:
+            db.rollback()
+    return {"video_id": video_id, "status": "new", "position_seconds": 0}
+
+
 @router.post("/videos/{video_id}/progress")
 def set_video_progress(
     video_id: str,