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/components/Feed.tsx

@@ -1,7 +1,7 @@
 import { useEffect, useRef, useState } from "react";
 import { useInfiniteQuery, useQuery, useQueryClient } from "@tanstack/react-query";
 import { api, type FeedFilters, type Video } from "../lib/api";
-import { toast } from "../lib/notifications";
+import { notify } from "../lib/notifications";
 import VideoCard from "./VideoCard";
 
 const PAGE = 60;
@@ -73,7 +73,18 @@ export default function Feed({
       .then(() => qc.invalidateQueries({ queryKey: ["feed"] }))
       .catch(() => {});
     if (status === "hidden") {
-      toast("Video hidden", { label: "Undo", onClick: () => onState(id, "new") });
+      const v = (query.data?.pages ?? []).flatMap((p) => p.items).find((x) => x.id === id);
+      notify({
+        message: v?.title ? `Hidden “${v.title}”` : "Video hidden",
+        action: { label: "Undo", onClick: () => onState(id, "new") },
+        meta: {
+          kind: "video-hidden",
+          videoId: id,
+          title: v?.title ?? "this video",
+          channelId: v?.channel_id ?? "",
+          channelName: v?.channel_title ?? "",
+        },
+      });
     }
   }