fix: address reader-UI feedback (shorts, search, tags, login, UX)

- Shorts: confirm via youtube.com/shorts/<id> probe (SOCS cookie bypasses the
  consent redirect) instead of a 60s heuristic; concurrent probing, shorts_probed
  flag, scheduled refinement (migration 0005)
- Search: match title + channel name only (descriptions caused noisy results)
- Faceted tag filtering: AND across categories (language AND topic narrows),
  OR within a category; any/all toggle applies to topics
- Language detection: majority vote over individual titles (fixes misdetections
  like multipoleguy -> English; drops bogus Polish/Romanian)
- Login: drop forced consent so returning sign-in is quick (select_account)
- Feed cards: clickable channel name (opens channel), persistent saved badge,
  undo toast on hide, Hidden view to restore; tag chips show counts in tooltip

frontend/src/components/Toaster.tsx

@@ -0,0 +1,29 @@
+import { useSyncExternalStore } from "react";
+import { dismiss, getToasts, subscribe } from "../lib/toast";
+
+export default function Toaster() {
+  const toasts = useSyncExternalStore(subscribe, getToasts, getToasts);
+  return (
+    <div className="fixed bottom-4 right-4 z-50 flex flex-col gap-2">
+      {toasts.map((t) => (
+        <div
+          key={t.id}
+          className="bg-surface border border-border rounded-xl shadow-2xl px-4 py-3 flex items-center gap-4 animate-[fadeIn_0.15s_ease]"
+        >
+          <span className="text-sm">{t.message}</span>
+          {t.action && (
+            <button
+              onClick={() => {
+                t.action!.onClick();
+                dismiss(t.id);
+              }}
+              className="text-accent text-sm font-semibold hover:underline"
+            >
+              {t.action.label}
+            </button>
+          )}
+        </div>
+      ))}
+    </div>
+  );
+}