feat(web): in-place Cursor variant drill-down in model picker

[heavygee]

Summary

• Keep the Cursor model settings popover open after picking a multi-variant base model, then drill down in-place to variants instead of forcing a second reopen.
• Apply and preselect the default variant immediately on base pick, and sort variant rows with default first.
• Add a back affordance (← Models) to return to the full model list without dismissing, while variant pick still dismisses.

Why

Users selecting non-default Cursor models are in a minority power-user path. Requiring a second picker open to choose variants is slow and easy to miss on mobile; this change makes base -> variant selection a single continuous flow.

Scope

• Cursor picker UX only.
• No backend/schema changes.
• Includes tests for default-variant mapping and SessionChat model-change behavior.

Test plan

• bun typecheck
• cd web && bunx vitest run src/lib/cursorVariantPicker.test.ts src/lib/cursorModelOptions.test.ts src/lib/sessionChatCursorModel.test.ts
• Dogfood in driver soup on live Cursor sessions (base pick keeps popover open, variant pick closes, default variant first)

Closes #48

Made with Cursor

[github-actions]

Findings
None.

Summary
Review mode: initial
No issues found in the latest diff. Residual risk: I could not run the Bun typecheck/tests in this runner because bun is not available in PATH, so CI should remain the source of truth for the interaction tests and strict TS coverage.

Testing

• git diff --check
• Not run: bun typecheck (bun: command not found)

HAPI Bot

[github-actions]

Findings
None.

Questions
None.

Summary
Review mode: follow-up after new commits
No issues found in the latest full PR diff. Residual risk: I could not run Bun-based typecheck/tests in this runner because bun is not available in PATH; CI should remain the source of truth for strict TS and interaction coverage.

Testing

• git diff --check origin/main...HEAD
• Not run (automation): bun typecheck, bun run test (bun not available in PATH)

HAPI Bot