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

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 ## [Unreleased]
 
 - Diff/Chat: added a Last turn mode to the Diff view, and latest-turn changed-file chips in chat now open that snapshot while older turn chips stay read-only.
+- Chat: if sending a message times out or loses the connection after OpenCode accepted it, the app now keeps the sent message instead of rolling it back as failed.
+- Mobile: the native app can now check for OpenChamber app updates; Android shows a persistent download toast when an update is available.
+- Mobile: selecting local files from the composer now attaches the picked files even if the composer switches between compact and expanded layouts while the file picker is open.
+- Browser: links clicked inside an embedded browser tab now keep the tab on the navigated page instead of remounting the frame.
+- Context Panel: raw message rows now keep token and time columns aligned without showing shortened message IDs.
+- UI: closing the right sidebar after resizing no longer leaves stale width constraints behind.
 
 ## [1.14.1] - 2026-07-07