feat(messages): E2EE real-time messaging UI + system welcome

A Messages tab in the notification center (hidden for demo): set up secure
messaging with a passphrase (key generated + wrapped in-browser via WebCrypto,
stored non-extractable per device), unlock on other devices, then chat with
end-to-end encrypted, live-delivered messages. The server-authored Siftlode
welcome is readable before any key setup.

- lib/e2ee.ts: ECDH P-256 + HKDF + AES-GCM, PBKDF2-wrapped key, IndexedDB.
- lib/messagesSocket.ts: WebSocket client with backoff reconnect.
- Messages.tsx: key gate, conversation list with decrypted previews, directory,
  thread with client-side decrypt + encrypt-on-send. Unread feeds the nav badge.
  EN/HU/DE strings.

frontend/src/i18n/locales/en/messages.json

@@ -0,0 +1,29 @@
+{
+  "tab": "Messages",
+  "conversations": "Conversations",
+  "new": "New message",
+  "newTitle": "New message",
+  "empty": "No conversations yet. Start one with the New message button.",
+  "searchPeople": "Search people…",
+  "noPeople": "No one to message yet.",
+  "threadEmpty": "No messages yet — say hello.",
+  "writePlaceholder": "Write a message…",
+  "send": "Send",
+  "encrypted": "Encrypted message",
+  "encryptedHint": "End-to-end encrypted",
+  "cantDecrypt": "Can't decrypt",
+  "systemReadOnly": "This is an automated message.",
+  "setupTitle": "Set up secure messaging",
+  "setupBody": "Messages are end-to-end encrypted. Choose a passphrase to protect your key — it never leaves your device, so no one (not even an admin) can read your conversations.",
+  "setupWarn": "If you forget this passphrase, your message history can't be recovered.",
+  "setUp": "Set up",
+  "unlockTitle": "Unlock messaging",
+  "unlockBody": "Enter your message passphrase to unlock your conversations on this device.",
+  "unlock": "Unlock",
+  "passphrase": "Passphrase",
+  "passphraseConfirm": "Confirm passphrase",
+  "passTooShort": "Use at least 6 characters.",
+  "passMismatch": "The passphrases don't match.",
+  "wrongPass": "Wrong passphrase.",
+  "setupFailed": "Couldn't set up secure messaging. Please try again."
+}