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/runtime-auth.ts

@@ -1,3 +1,5 @@
+import { getActiveRelayTunnel } from '@/lib/relay/runtime-tunnel';
+
 type RuntimeAuthCredential =
   | { type: 'bearer'; token: string }
   | null;
@@ -201,11 +203,16 @@ const mintRuntimeUrlAuthToken = (apiBaseUrl?: string | null): Promise<string> =>
     if (credential?.type === 'bearer') {
       headers.set('Authorization', `Bearer ${credential.token}`);
     }
-    const response = await fetch(buildAuthUrl(apiBaseUrl, '/auth/url-token'), {
-      method: 'POST',
-      headers,
-      credentials: 'include',
-    });
+    // In relay mode the mint must ride the tunnel, not the network: there is no
+    // reachable network base URL. Same auth headers, same route, tunneled.
+    const relay = getActiveRelayTunnel();
+    const response = relay
+      ? await relay.fetch('/auth/url-token', { method: 'POST', headers })
+      : await fetch(buildAuthUrl(apiBaseUrl, '/auth/url-token'), {
+          method: 'POST',
+          headers,
+          credentials: 'include',
+        });
     if (!response.ok) {
       if (generation === runtimeAuthGeneration) {
         clearRuntimeUrlAuthToken();