fix(app): identify desktop in Console device auth (v1)

[opencode-agent]

Summary

Set client_id=opencode-desktop directly in the existing V1 provider-auth handler before showing the Console verification link. Only applies to OpenCode sign-in from Desktop.

• One existing file changed: 7 added lines.
• No new utilities or test files.
• Web clients, other providers, device codes, URL targets, and fragments are unchanged.
• This is the browser verification query only; device-code issuance, polling, refresh, and their OAuth client binding are unchanged.

Validation

• App package typecheck, Prettier, and diff checks pass.
• Temporary smoke harness executing the actual V1/V2 auth handlers passes for Desktop, web, and other-provider cases. No harness files are included in the PR.

Console handling

Paired with anomalyco/opencode-console#1983.

Requested by: @vimtor (victor via Slack)