feat: per-row default stars and adaptive picker placement

[auchan]

Summary

Follow-up to the reusable option picker:

• per-row default controls: non-default model/thinking rows show a hollow ☆ on hover that sets that row as default without selecting/closing; the current default model shows a solid ★ that clears the default on click
• recent model memory moved from webview localStorage into extension workspaceState so it survives reloads
• search box only appears for lists likely to overflow the panel height
• picker placement adapts to the clicked status item (topLeft for left-side items, topRight otherwise) instead of always clamping to the left edge

Validation

• bun run check-types
• bun run lint
• bun run compile-tests
• option-picker helpers + protocol tests (25 passing)
• git diff --check