fix(ui): notification UX — outside-click close, toast countdown, hide entry actions

- NotificationCenter closes on outside click (document listener, not an overlay
  that the blurred header trapped) and no longer needs a second bell click.
- Toasts show a level-colored countdown bar and auto-dismiss faster (6s default).
- Hidden-video notifications carry structured meta so the center offers an in-app
  'Find in feed' (jump to that channel's hidden videos) and a one-click 'Unhide',
  working even after a reload when the live Undo callback is gone.

frontend/src/index.css

@@ -115,6 +115,16 @@ html[data-scheme="youtube"][data-theme="light"] {
   --accent-fg: #ffffff;
 }
 
+/* Toast auto-dismiss countdown bar (shrinks left-to-right to zero) */
+@keyframes toastbar {
+  from {
+    transform: scaleX(1);
+  }
+  to {
+    transform: scaleX(0);
+  }
+}
+
 /* Thin, theme-aware scrollbars */
 * {
   scrollbar-color: var(--border) transparent;