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

packages/mobile/android/app/src/main/AndroidManifest.xml

@@ -1,11 +1,17 @@
 <?xml version="1.0" encoding="utf-8" ?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
+    <!-- usesCleartextTraffic: OpenChamber connects to user-hosted servers over
+         plain http:// on the local network (LAN transport). Android blocks all
+         cleartext HTTP by default (targetSdk >= 28), which silently failed every
+         LAN probe and forced Android onto relay-only. This mirrors the iOS ATS
+         exceptions (NSAllowsArbitraryLoadsInWebContent + NSAllowsLocalNetworking). -->
     <application
         android:allowBackup="true"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
+        android:usesCleartextTraffic="true"
         android:theme="@style/AppTheme">
         <activity
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|smallestScreenSize|screenLayout|uiMode|navigation"