feat: add private relay for end-to-end-encrypted remote access (#2087)

Adds OpenChamber Relay — an opt-in way to reach an instance from a phone,
browser, or another desktop from anywhere, with no open inbound ports, no
tunnel, and no shared LAN. The instance dials outbound to a relay; all app
traffic (HTTP, the event stream, terminal, dictation) is multiplexed and
encrypted through a single connection per client, so the relay only ever
forwards opaque ciphertext.

Transport
- End-to-end-encrypted channel over WebCrypto (ECDH P-256 -> HKDF ->
  AES-256-GCM) with a capability-negotiated handshake and a small
  HTTP/SSE/WebSocket multiplexing protocol. A byte-compatible JS host mirror
  is cross-checked by tests.
- Host: outbound connection manager, per-client tunnel dispatcher to the local
  server over loopback, reuse of the existing instance identity key, and
  management routes. Disabled by default; explicit opt-in.
- Client: plugs into the existing runtime layer (runtime-fetch/-url/-switch/
  -auth, event pipeline, terminal, dictation) so features work over the relay
  unchanged; direct-URL and Electron realtime-proxy paths are untouched.

Pairing & UX
- Relay section in Settings -> Remote Instances (live status, QR/link pairing,
  revocation via the existing client-token list) and the mobile connect flow.
- Frame batching and idle-gated keepalive keep tunnel message volume low
  without affecting streaming smoothness.

Security
- The tunnel is transport only; the server authenticates every tunneled
  request exactly as for a direct remote client.
  fragments only. The relay stores no keys, tokens, or payloads.

Operability
- The endpoint can be pinned to a self-hosted rel
  paired clients inherit it from the offer automatically.
- Relay module DOCUMENTATION.md and a relay-trans
  invariants that future WebSocket/streaming changes must follow.

The relay transport is complete and tested; the UI for enabling and pairing
is gated behind openchamber_relay_gate and stays

packages/ui/src/lib/settings/search.ts

@@ -1,6 +1,7 @@
 import type { I18nKey } from '@/lib/i18n/store';
 import type { SettingsPageSlug, SettingsRuntimeContext } from './metadata';
 import { getSettingsPageMeta } from './metadata';
+import { RELAY_UI_ENABLED } from '@/lib/relay/gate';
 
 interface SettingsSearchItem {
   id: string;
@@ -426,6 +427,15 @@ const SETTINGS_SEARCH_ITEMS: readonly SettingsSearchItem[] = [
     keywords: ['pairing link', 'client token', 'connect desktop', 'remote access'],
     isAvailable: (ctx) => !ctx.isVSCode,
   },
+  {
+    id: 'remote-instances.relay',
+    page: 'remote-instances',
+    titleKey: 'settings.remoteInstances.relay.title',
+    descriptionKey: 'settings.remoteInstances.relay.description',
+    keywords: ['relay', 'pairing', 'no ports', 'end-to-end encrypted', 'remote access', 'connect from anywhere'],
+    // Gated by openchamber_relay_gate until the relay UI ships publicly.
+    isAvailable: (ctx) => !ctx.isVSCode && RELAY_UI_ENABLED,
+  },
   {
     id: 'remote-instances.direct-hosts',
     page: 'remote-instances',