🤖 fix: restore DirectoryPickerModal in ProjectCreateModal

PR #637 accidentally removed the DirectoryPickerModal integration that was
added in PR #682. This restores:

- Desktop mode: native pickDirectory() dialog
- Browser mode: web-based DirectoryPickerModal component
- Conditional Browse button based on platform capabilities
- Theme-aware styling (text-foreground instead of hardcoded text-white)

Also adds ProjectCreateModal.stories.tsx with 8 Storybook tests covering:
- Basic modal rendering
- Typing path manually
- Browse button opening directory picker
- Directory navigation
- Path selection flow
- Full end-to-end flow
- Cancel behavior
- Validation errors

Author: ibetitsmike