feat: Add PRD with OCR and translation prd

[hubo1989]

Add product requirements document for ScreenTranslate app:

• US-001: Base architecture (menu bar + screenshot) - already implemented
• US-002: Local OCR using Vision framework
• US-003: Local translation using Apple Translation API
• US-004/005: Overlay rendering (in-place and below modes)
• US-006/007: Settings panel (engine selection + language config)
• US-008/009: Optional PaddleOCR and MTranServer integration
• US-010: Translation history
• US-011: First-launch onboarding

Architecture prioritizes local processing (Vision + Apple Translation) with optional external engines for advanced users.

Summary

Brief description of the changes in this PR.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #(issue number)

Changes Made

• Change 1
• Change 2
• Change 3

Testing

Describe the tests you ran to verify your changes:

• Full screen capture works
• Selection capture works
• Annotation tools function correctly
• Undo/redo works
• Save and copy to clipboard work
• Settings persist correctly
• No memory leaks observed

Screenshots

If applicable, add screenshots showing the changes.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code where necessary
• I have updated the documentation accordingly
• My changes generate no new warnings
• I have tested on macOS 13+
• I have tested with multiple displays (if applicable)