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/modules.ts

@@ -6,7 +6,6 @@ import {
   Download,
   Home,
   ListVideo,
-  MessageSquare,
   ScrollText,
   FileSearch,
   Settings,
@@ -90,14 +89,6 @@ const MODULES: readonly ModuleDef[] = [
     icon: Bell,
     hasBadge: true,
   },
-  {
-    id: "messages",
-    labelKey: "messages.navLabel",
-    railKey: "nav.rail.messages",
-    icon: MessageSquare,
-    hasBadge: true,
-    gate: notDemo,
-  },
   {
     id: "downloads",
     labelKey: "downloads.navLabel",