feat(messages): R4 S2 — forgotten-passphrase reset for E2EE

The set-once key guard made a lost passphrase a permanent wall (409 on re-setup,
KeyGate offered only unlock/setup). Add an escape hatch:

- POST /api/messages/keys/reset drops the user's MessageKey so a new passphrase
  can be set. Because every conversation key is derived from the keypair,
  replacing it orphans all stored ciphertext in both directions, so the now-dead
  `user` messages are deleted too (clean slate; system messages untouched).
  Password accounts must re-verify (a hijacked session must not silently reset
  messaging and read future messages); Google-only accounts rely on the session
  + the client danger-confirm. Rate-limited per user.
- KeyGate gains a "Forgot passphrase?" path in unlock mode → a danger reset view
  (spells out the permanent history loss; a current-password field for password
  accounts) → resets and returns to a fresh setup.
- e2ee.resetLocal wipes this device's key + derived conversation keys + the
  IndexedDB copy; api.resetMessageKey; HU/EN strings.

frontend/src/lib/api.ts

@@ -1623,6 +1623,14 @@ export const api = {
   messageMyKey: (): Promise<MyKeyBundle> => req("/api/messages/keys/me"),
   setupMessageKey: (k: KeySetup): Promise<{ configured: boolean }> =>
     req("/api/messages/keys", { method: "POST", body: JSON.stringify(k) }),
+  // Forgotten-passphrase reset: drops the server key (and the now-undecryptable history) so a new
+  // passphrase can be set. Password accounts must pass their current password; errors shown inline.
+  resetMessageKey: (currentPassword?: string): Promise<{ configured: boolean }> =>
+    req(
+      "/api/messages/keys/reset",
+      { method: "POST", body: JSON.stringify({ current_password: currentPassword ?? null }) },
+      { quiet: true },
+    ),
   messagePublicKey: (userId: number): Promise<{ user_id: number; public_key: string }> =>
     req(`/api/messages/keys/${userId}`),
   messageUsers: (): Promise<MessageUser[]> => req("/api/messages/users"),