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/ui/src/components/views/SettingsWindow.tsx

@@ -54,6 +54,9 @@ export const SettingsWindow: React.FC<SettingsWindowProps> = ({ open, onOpenChan
               'transition-all duration-150 ease-out',
               'data-[starting-style]:opacity-0 data-[starting-style]:scale-[0.98]',
               'data-[ending-style]:opacity-0 data-[ending-style]:scale-[0.98]',
+              // Dim this window when a nested dialog (e.g. "Add a device") opens
+              // on top of it, mirroring how the page behind a dialog is dimmed.
+              'data-[nested-dialog-open]:brightness-[0.55] dark:data-[nested-dialog-open]:brightness-[0.4]',
             )}
           >
             <Dialog.Description id={descriptionId} className="sr-only">