Add screenshot mode, dark mode support, and integrate logo

Features:
- Screenshot mode (--screenshot-mode) for automated UI captures
- Theme override (--theme=light|dark|system) for testing
- FDA override (--force-no-fda) to test permission screen
- All 12 screenshots: 6 screens × 2 themes

UI improvements:
- Dark mode now properly follows system preference
- Fixed hardcoded colors to use CSS variables
- Added logo to header from project/logos/
- Updated all Tauri app icons from new logo
- Updated footer copy: "Only text is uploaded — no images or videos"

Technical:
- Added xcap crate for cross-platform window capture
- Added screenshot.rs module with capture_window()
- Frontend cycles through screens in screenshot mode
- CLI args parsed via clap
- Fixed file-length check to skip binary files

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ndbroadbent