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/sourceSize.contract.test.ts

@@ -52,7 +52,6 @@ const COMPLEX: Record<string, number> = {
   "components/Home.tsx::Home": 34,
   "components/Home.tsx::useHomeCardMenu": 21,
   "components/IssueComposer.tsx::ComposerBody": 34,
-  "components/KeyGate.tsx::KeyGate": 22,
   "components/NotificationsPanel.tsx::NotificationRow": 28,
   "components/OnboardingWizard.tsx::OnboardingWizard": 21,
   "components/PersonalViewsModal.tsx::PersonalViewsModal": 22,