feat: configurable track forward, track to end, skip-and-mark (REQ-189)

Track 5 Frames becomes Track Forward with a 1-100 count input, remembered in
localStorage (default 5). A separate -> End action runs to the last frame
behind a confirm and can be cancelled mid-flight.

Frames that already hold a shape of the tracked class are skipped instead of
re-annotated: hidden and dimmed shapes count as present. Skips are reported in
the result banner and marked with a dot plus a class name on the filmstrip
thumbnail. frames() now returns the class ids each frame holds, the same query
row, no migration.

Chaining and seed growth stay out: every frame is still seeded from the drawn
box only.

backend/batches.py

@@ -137,11 +137,16 @@ def frames(batch_id: int) -> List[dict]:
     with db.cursor() as cur:
         cur.execute(
             """SELECT f.*, (SELECT COUNT(*) FROM annotations a WHERE a.frame_id = f.id)
-                        AS annotation_count
+                        AS annotation_count,
+                        (SELECT GROUP_CONCAT(DISTINCT a.class_id) FROM annotations a
+                          WHERE a.frame_id = f.id) AS class_ids
                FROM frames f WHERE f.batch_id = ? ORDER BY f.idx""",
             (batch_id,),
         )
-        return [dict(row) for row in cur.fetchall()]
+        rows = [dict(row) for row in cur.fetchall()]
+    for row in rows:
+        row["class_ids"] = [int(v) for v in row["class_ids"].split(",")] if row["class_ids"] else []
+    return rows
 
 
 def frame_path(frame_id: int) -> Optional[str]: