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.

backend/tests/test_ratelimit.py

@@ -57,6 +57,19 @@ def test_reset_clears_every_key():
     assert lim.allow("b") is True
 
 
+def test_exhausted_reports_a_full_window_without_counting_an_attempt():
+    # The asking half of a limit that counts only some outcomes (a share link's unlock counts only
+    # wrong passwords toward its ceiling): asking must never use up the window it asks about.
+    lim = RateLimiter(max_events=2, window_seconds=300)
+
+    for _ in range(5):
+        assert lim.exhausted("link") is False
+    assert lim.allow("link") is True
+    assert lim.exhausted("link") is False
+    assert lim.allow("link") is True
+    assert lim.exhausted("link") is True
+
+
 def test_expired_attempts_fall_out_of_the_window():
     # A zero-length window means every previous attempt is already outside it.
     lim = RateLimiter(max_events=1, window_seconds=0)