feat!: remove direct messaging (chat)

E2EE direct messages, the chat dock, the Messages module, the message
WebSocket and its realtime registry, and the messages / message_keys
tables (migration 0097; the downgrade restores the empty schema). Prod
held two welcome notes, no message between people and one key — never
used (owner decision, E34). WebSocket now has no owner module: the lint
bans it everywhere. Privacy policy updated: it no longer promises E2EE
for a feature that is gone.

frontend/src/lib/pageMeta.test.ts

@@ -11,7 +11,6 @@ const EXPECTED: Record<Page, string> = {
   channels: "header.channelManager",
   playlists: "header.account.playlists",
   notifications: "inbox.navLabel",
-  messages: "messages.navLabel",
   downloads: "downloads.navLabel",
   stats: "header.usageStats",
   plex: "plex.navLabel",