feat(web): show recent git branches in new session dropdown

[R44VC0RP]

Summary

• Add recent git branches to the "New session" dropdown for quick branch switching
• Users can now see and checkout recent branches without leaving the web UI

Changes

• Backend: Added GET /vcs/branches endpoint to list recent branches sorted by commit date
• Backend: Added POST /vcs/checkout endpoint to switch branches
• Frontend: Updated session-new-view dropdown to show up to 6 recent branches
• Frontend: Handle branch selection with checkout API and toast notifications
• SDK: Regenerated with new types (BranchInfo, CheckoutInput, CheckoutResult)

Testing

1. Start API server: cd packages/opencode && bun dev -- serve --port 5556
2. Start Vite: cd packages/app && VITE_API_URL=http://127.0.0.1:5556 bun dev
3. Open http://localhost:3001, click "New session"
4. Click branch dropdown - should show recent branches
5. Select a different branch - should checkout and show toast

Notes

• The pre-existing TypeScript "type instantiation excessively deep" error in server.ts was triggered by adding more routes. This is a known issue (see TODO at line 77) and exists independently of these changes.