ProviderOnboarding: Maximum update depth loop on clean first-run profile

[GoDiao]

Summary

While verifying #39, @yhay81 hit a pre-existing render loop: with a fresh/empty WebView2 profile (no provider configured yet), ProviderOnboarding enters a Maximum update depth exceeded loop and the app stays stuck at Loading....

This is not caused by #39 ? it reproduces on the dev baseline. Splitting it out so the locale work could land cleanly.

Impact

• Blocks the first-run onboarding flow on a clean profile.
• Made it impossible to complete a manual Settings-switch / restart click-through during fix: detect system locale on first launch #39 review (that path was covered by focused unit tests instead).

Repro

1. Launch bun run tauri dev with an isolated, empty WebView2 profile (no provider / API key configured).
2. App renders Loading... and never advances.
3. Console shows React Maximum update depth exceeded originating from ProviderOnboarding.

Likely area

desktop/src/components/** around ProviderOnboarding ? probable useEffect / setState cycle without a stable dependency guard. Needs someone to trace the effect dependency chain.

Credit

Surfaced by @yhay81 during #39 verification.