🤖 fix: restore DirectoryPickerModal in ProjectCreateModal (#756)

PR #637 accidentally removed the DirectoryPickerModal integration added
in PR #682.

## Changes

**ProjectCreateModal.tsx:**
- Restored desktop mode: native `pickDirectory()` dialog
- Restored browser mode: web-based `DirectoryPickerModal` component  
- Browse button conditionally shown based on platform capabilities
- Theme-aware styling (`text-foreground` instead of hardcoded
`text-white`)

**ProjectCreateModal.stories.tsx (new):**
- 8 Storybook tests covering directory picker flow
- Tests for navigation, selection, cancel, and validation errors

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Author: ibetitsmike