fix(weixin): single-source QR login and non-blocking Settings refresh

[Gucc111]

Summary

• Make WeixinChannel the single source of QR login — the UI server no longer calls weixin-ilink.loginWithQR() directly, eliminating the dual-login race condition.
• Add prepare_weixin_login gateway RPC so the Settings page can trigger a backend channel start/restart without importing weixin-ilink.
• Fix Settings auto-refresh unmounting the QR code: useGatewayStatus now does silent refresh (no full-page loading spinner) once initial data is loaded.
• Write qrUrl into runtime-status.json from WeixinChannel.onQRCode, so the UI reads the QR code from runtime state instead of maintaining its own login task.
• Use requestedAt timestamp to ignore stale failed/expired runtime errors from previous attempts.

Test plan

• npm test — 58 tests pass (includes new regression tests for begin route, runtime QR persistence, stale error filtering, and protocol RPC exposure)
• npm --workspace ui run build — succeeds
• Manual: start PilotDeck with adapters.weixin.enabled: false, click WeChat QR login in Settings → should show "preparing" then display QR code
• Manual: with an old failed runtime state, click QR login again → should restart the channel, not show the stale error
• Manual: QR code stays visible during 3-second auto-refresh cycles (no unmount/remount)

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