Add offline indicator, surprise me button, and continue watching on channel

- Offline banner in nav when backend is unreachable (network error, not 4xx)
- GET /channels/{id}/random — picks random unwatched video, navigates to watch
- GET /channels/{id}/in-progress — videos with >30s progress, not yet watched
- Channel page: 'Surprise me' button (desktop + mobile) navigates to random video
- Channel page: 'Continue watching' row above video list when in-progress videos exist

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/channels.py

@@ -865,6 +865,66 @@ def _search_channel_task(channel_id: int, youtube_channel_id: str, q: str, user_
         db.close()
 
 
+@router.get("/{channel_id}/random", response_model=VideoOut)
+def get_random_channel_video(
+    channel_id: int,
+    unwatched_only: bool = True,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    _get_channel_or_404(db, channel_id)
+    unwatched_clause = "AND COALESCE(uv.watched, 0) = 0" if unwatched_only else ""
+    row = db.execute(
+        text(f"""
+            SELECT v.id, v.youtube_video_id, v.title, v.thumbnail_url,
+                   v.duration_seconds, v.published_at, v.view_count,
+                   c.id AS channel_id, c.name AS channel_name, c.youtube_channel_id AS channel_youtube_id,
+                   COALESCE(uv.downloaded, 0) AS is_downloaded,
+                   COALESCE(uv.watched, 0) AS is_watched,
+                   COALESCE(uv.queued, 0) AS queued
+            FROM videos v
+            JOIN channels c ON v.channel_id = c.id
+            LEFT JOIN user_videos uv ON v.id = uv.video_id AND uv.user_id = :user_id
+            WHERE v.channel_id = :channel_id {unwatched_clause}
+            ORDER BY RANDOM()
+            LIMIT 1
+        """),
+        {"user_id": current_user.id, "channel_id": channel_id},
+    ).mappings().first()
+    if not row:
+        raise HTTPException(status_code=404, detail="No videos found")
+    return VideoOut(**dict(row))
+
+
+@router.get("/{channel_id}/in-progress", response_model=list[VideoOut])
+def get_channel_in_progress(
+    channel_id: int,
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    _get_channel_or_404(db, channel_id)
+    rows = db.execute(
+        text("""
+            SELECT v.id, v.youtube_video_id, v.title, v.thumbnail_url,
+                   v.duration_seconds, v.published_at, v.view_count,
+                   c.id AS channel_id, c.name AS channel_name, c.youtube_channel_id AS channel_youtube_id,
+                   COALESCE(uv.downloaded, 0) AS is_downloaded,
+                   COALESCE(uv.watched, 0) AS is_watched,
+                   COALESCE(uv.queued, 0) AS queued
+            FROM videos v
+            JOIN channels c ON v.channel_id = c.id
+            JOIN user_videos uv ON v.id = uv.video_id AND uv.user_id = :user_id
+            WHERE v.channel_id = :channel_id
+              AND uv.watch_progress_seconds > 30
+              AND COALESCE(uv.watched, 0) = 0
+            ORDER BY uv.watch_progress_seconds DESC
+            LIMIT 6
+        """),
+        {"user_id": current_user.id, "channel_id": channel_id},
+    ).mappings().all()
+    return [VideoOut(**dict(r)) for r in rows]
+
+
 @router.post("/{channel_id}/follow", status_code=status.HTTP_204_NO_CONTENT)
 def follow_channel(
     channel_id: int,