Accounts Phase 4: prefs sync + account/settings panel

- Prefs sync: GET/PUT /api/prefs store Calm Filters/Boundaries on the account.
  On sign-in the client adopts the account's prefs if present, else seeds them
  from the device; every change PUTs to the account so tuning follows you across
  devices. (Login side-effects run under untrack so browsing doesn't re-trigger.)
- Account panel: GET /api/account (email, connected sign-in methods, saved count,
  active sessions); Export my data (GET /api/account/export → JSON download);
  Sign out everywhere (revoke all sessions); Delete account (cascades to all
  account data) with an inline confirm. Reachable from You → Account.

Deferred to a follow-up: link/unlink a provider (OAuth link-mode) and per-session
revoke. 118 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/auth.svelte.js

@@ -59,7 +59,13 @@ export async function logout() {
   try {
     await postJSON('/api/auth/logout', {});
   } finally {
-    auth.user = null;
-    savedIds.clear();
+    clearLocal();
   }
 }
+
+// Clear client auth state without a logout call (cookie already cleared server-side,
+// e.g. after delete-account or sign-out-everywhere).
+export function clearLocal() {
+  auth.user = null;
+  savedIds.clear();
+}