feat(relay): add manual session node selection

[lulu0119]

What changed

• let users measure and manually choose the Japan primary or China relay before each session
• add a Settings default node while keeping active sessions pinned
• disclose the China relay's direct-IP HTTP risk in the UI
• reduce the China deployment to an API-only relay on port 8443 and remove Caddy/Cloudflare paths
• configure the primary server with the China relay origin during production deployment

Why

Users need direct control over user-to-node latency. The China server cannot expose ports 80/443, so the relay uses a direct IP and port without pretending the connection is encrypted.

Validation

• typecheck: app-shared, pages, web, desktop, playground, api
• API tests: 22 passed
• app-shared tests: 24 passed
• Web production build passed
• China relay health and latency endpoints verified on port 8443