feat(messages): standalone Messages module + floating chat dock

Move messaging out of the notification center into its own left-nav module
(own page, own unread badge), so a reload returns to it and notifications stay
separate. Add a Messenger-style floating dock (bottom-right): pop a conversation
out from the page, keep chatting across navigation, minimise (rollup) or close.

- Messages is now a routed page; NotificationsPanel reverts to inbox-only and the
  nav badge no longer mixes in messages.
- Extract shared KeyGate + ChatThread (used by both the page and dock windows);
  e2ee exposes a shared unlock subscription so page and dock agree.
- ChatDock (always mounted for human users) owns the app-wide live-message
  subscription and per-device key restore. EN/HU/DE strings.

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

@@ -1,5 +1,10 @@
 {
   "tab": "Messages",
+  "navLabel": "Messages",
+  "popOut": "Open in pop-out",
+  "expand": "Expand",
+  "minimize": "Minimize",
+  "close": "Close",
   "conversations": "Conversations",
   "new": "New message",
   "newTitle": "New message",