feat: pairing v2 — one-tap trusted devices over LAN and private relay (#2103)

Reworks how devices connect to an OpenChamber server, end to end.

Pairing v2:
- One-time pairing links/QR codes (openchamber://connect?v=2) carrying a set of transport candidates (LAN/tunnel/relay) and a single-use secret redeemed server-side; no tokens embedded in links
- Add-a-device dialog written for first-time users: intent-based transport choice (Anywhere / Home network only / This computer only) with plain-language descriptions, transparent fallback checkboxes, server-authoritative LAN detection, high-res QR dialog
- Private relay folded into pairing as a transport candidate with a demand-driven lifecycle (enables when a relay device is paired, disables when none remain)

Multi-transport devices:
- A saved device holds all its transports and one token; mobile re-probes on connect, resume, and network change and hot-switches LAN<->relay seamlessly (no re-pairing, no remount, session preserved)
- Desktop can import relay pairing links, switch to relay hosts through the E2EE tunnel, and restore a relay default host after relaunch

Device management:
- Device list (web + desktop) shows live per-device connectivity with the active transport (Connected - Local network / Relay) and platform badges (iOS/Android/macOS/Windows/Linux)
- One physical device = one record: stable per-install dedupe keys across pairing and password re-login; typed pairing label names the device, paired devices name the connection by the issuing server hostname
- Trusted desktop-local client manages all devices (list, revoke, clear revoked); relay host reaps dead client sockets after 3 missed keepalives

Android:
- LAN transport unblocked (cleartext + mixed content, mirroring iOS ATS exceptions); resume re-probe retries through network flux and silently auto-reconnects from a disconnected state

CHANGELOG.md

@@ -4,6 +4,8 @@ All notable changes to this project will be documented in this file.
 
 ## [Unreleased]
 
+- Remote access: a new private relay lets you reach your instance from anywhere — no open ports and no third-party tunnel. Enable it in Settings → Remote Instances, and pairing links (QR or `openchamber connect-url`) can carry the relay as a fallback so a device off your network connects over an end-to-end-encrypted tunnel.
+- Pairing: device pairing links are now single-use, expiring codes redeemed on connect instead of embedding a long-lived token in the QR.
 - 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.