fix(ui): portal tooltips, opaque glass, settings rail+stable height, notif test, channel help

- Tooltip: render in a portal with fixed positioning + edge-flip so hints are never
  clipped by overflow/stacking ancestors (fixes mispositioned/hidden bubbles app-wide).
- Glass: raise opacity so overlay menus/panels stay readable over content.
- SettingsPanel: vertical tab rail (no wrapping/jumping), content grid-stacked so the
  panel sizes to the tallest tab (stable height) and floats to its content height.
- Notifications: the test toast is now a normal auto-dismissing toast (with countdown
  bar) that also plays the sound via a new force-sound flag.
- Channel manager: explain priority/tags/hide and what 'Sync subscriptions' does;
  add a 'Channel priority' feed sort so priority is actually meaningful.

backend/app/routes/feed.py

@@ -208,6 +208,8 @@ SORTS = {
     "duration_asc": Video.duration_seconds.asc().nulls_last(),
     "title": func.lower(Video.title).asc().nulls_last(),
     "subscribers": Channel.subscriber_count.desc().nulls_last(),
+    # Your per-channel priority (set in the channel manager), newest first within a tier.
+    "priority": Subscription.priority.desc(),
 }
 
 
@@ -222,10 +224,15 @@ def get_feed(
     db: Session = Depends(get_db),
 ) -> dict:
     query, _status = _filtered_query(db, user, **params)
-    order = SORTS.get(sort)
-    if order is None and sort == "shuffle":
-        order = func.md5(func.concat(Video.id, str(seed)))
-    query = query.order_by(order if order is not None else SORTS["newest"])
+    if sort == "priority":
+        query = query.order_by(
+            Subscription.priority.desc(), Video.published_at.desc().nulls_last()
+        )
+    else:
+        order = SORTS.get(sort)
+        if order is None and sort == "shuffle":
+            order = func.md5(func.concat(Video.id, str(seed)))
+        query = query.order_by(order if order is not None else SORTS["newest"])
 
     rows = db.execute(query.offset(offset).limit(limit + 1)).all()
     has_more = len(rows) > limit