feat(header): clarify video counts and hide idle pause button

Show 'N yours / M total' in the header (your subscriptions vs. the whole shared
catalog) with a tooltip, backed by a new total_videos field on /sync/my-status.
The admin pause button now only appears when there's sync work to pause; Resume
still shows whenever sync is paused.

backend/app/routes/sync.py

@@ -152,6 +152,9 @@ def my_status(
             ),
         )
     )
+    # Whole-catalog total (every video any user's subscriptions have pulled in), so the
+    # header can show "yours" against the shared library size.
+    total_videos = db.scalar(select(func.count(Video.id)))
     return {
         "channels_total": total,
         "channels_details_synced": details_synced,
@@ -163,6 +166,7 @@ def my_status(
         "deep_pending_count": deep_eta["channels_pending"],
         "deep_eta_seconds": deep_eta["eta_seconds"],
         "my_videos": my_videos or 0,
+        "total_videos": total_videos or 0,
         "quota_used_today": quota.units_used_today(db),
         "quota_remaining_today": quota.remaining_today(db),
         "paused": state.is_sync_paused(db),