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.

frontend/src/components/Sidebar.tsx

@@ -41,6 +41,7 @@ const SORTS = [
   { id: "duration_asc", label: "Shortest" },
   { id: "title", label: "Name (A–Z)" },
   { id: "subscribers", label: "Channel subscribers" },
+  { id: "priority", label: "Channel priority" },
   { id: "shuffle", label: "Surprise me" },
 ];