fix: review round 3 — a new tab keeps its account's notices, a revoked tab account lands on the default

Deep-review round 3 over main..improvement/watch-background-audio.

Account boot
- The notification history and settings, and the hints toggle, are read
  when their modules load, which in a new tab (bookmark, typed URL) is
  before BootGate pins the account: they read as nobody's, and the tab's
  first notice replaced the account's stored history (up to 100 entries)
  with a one-item list. They now read again when the tab is pinned
  (onAccountPinned, run by setActiveAccount); a notice raised before the
  pin stays, after the history.
- A tab pinned to an account still in this browser's wallet but whose
  session ended (suspended, deleted, or revoked by "log out everywhere")
  got "signed out" from /api/me while the default account was still
  signed in, and signing in again as anyone else left it there. A refused
  per-tab account now leaves the wallet, and the probe answers as the
  default, so the tab reloads onto it.

Share page (/watch)
- The password unlock is limited per viewer (link + client IP), under a
  per-link ceiling that counts only wrong passwords. Keyed on the link
  alone, ten wrong guesses every five minutes locked out every viewer,
  the ones with the password included. client_ip moves to app.ratelimit
  so the public routes can key on it.
- Its failure messages and the load-failure screen are announced
  (role=alert, the password input described by its error). The sign-in,
  passphrase, password-change and setup form errors get role=alert too.

Comments that still described removed machinery are corrected: App's
collapse flags, SettingsPanel's Save bar, ConfigPanel, authed_format_sig.

Tests: the account's history and hints across the pin; a revoked and a
suspended pinned account; the per-viewer limit and the ceiling; the
limiter's exhausted(); the share page's announced failures; a share
link's feed filters persisted. Each mutation-checked RED for its own
reason.

frontend/src/components/KeyGate.tsx

@@ -106,7 +106,11 @@ export default function KeyGate({ meId, compact = false }: { meId: number; compa
             className={inputCls}
           />
         )}
-        {err && <div className="text-sm text-danger">{err}</div>}
+        {err && (
+          <div role="alert" className="text-sm text-danger">
+            {err}
+          </div>
+        )}
         <div className="flex gap-2">
           <Button variant="outline" onClick={cancelReset} disabled={busy}>
             {t("common.cancel")}
@@ -153,7 +157,11 @@ export default function KeyGate({ meId, compact = false }: { meId: number; compa
           className={inputCls}
         />
       )}
-      {err && <div className="text-sm text-danger">{err}</div>}
+      {err && (
+        <div role="alert" className="text-sm text-danger">
+          {err}
+        </div>
+      )}
       <Button variant="primary" size="md" onClick={submit} disabled={busy || !pass}>
         <Lock className="w-4 h-4" />
         {configured ? t("messages.unlock") : t("messages.setUp")}