Add a new Tauri command and front-end flow to create PRs/MRs.
The backend builds and validates provider-specific payloads and
endpoints, sends creation requests, and parses responses into the
app's review model. A Svelte dialog and a JS wrapper wire the UI to the
command so users can create requests from the Review Center.
- Implement provider payload and endpoint logic with unit tests
- Expose create_integration_review_request as a Tauri command
- Add CreateReviewDialog UI and integrate a creation button in Review Center
