feat(cli): pair with direct server links - #49971

Summary

• Make opencode pair print and encode a direct /connect URL for the advertised server.
• Keep credentials in the URL fragment so they are not included in the initial HTTP request.
• Let the app QR scanner accept direct pairing URLs as well as legacy raw JSON and query links.
• Document the direct pairing link.

Builds on #49291, now merged into v2.

Link format

http://<advertised-server>/connect#<base64url({ username: "opencode", password })>

The /connect page derives the server address from the URL origin, saves the credentials, and then starts authenticated requests. opencode pair --url <url> uses that URL as the direct link origin.

Validation

• App pairing URL and scanner tests — 11 passed.
• App typecheck — passed.
• CLI typecheck — passed.
• CLI web-shell/authentication tests — 2 passed.
• Documentation typecheck and generated-file check — passed.