docs: Add documentation and localization for Sprint 3

[Co0ob1iee]

Localization Updates:

• Added 50+ new localization keys to Resources.resx
• Categories: Toast, UploadHistory, Editor, Overlay
• Keys for all Sprint 3 features and UI elements

Toast Notification Keys:

• Toast types: Success, Info, Warning, Error
• Messages: Screenshot captured, Upload complete, URL copied
• Standard notification messages

Upload History Keys:

• Window title and headers
• Column headers and labels
• Action tooltips (Copy URL, Reupload, Open in browser)
• Status messages (Success, Failed, Uploading)
• Empty state messages

Editor Improvement Keys:

• Zoom controls (In, Out, Fit)
• Redo functionality
• Tool properties panel
• Status bar elements (Image size, Edits, Mode)
• Tool names (Marker, Rectangle, Arrow, Blur, Text)
• Quick actions and color picker

Overlay Improvement Keys:

• Help panel title and shortcuts
• Keyboard shortcut descriptions (ESC, Enter, F1, Space)
• Action buttons (Capture, Cancel)
• Dimension display format

Comprehensive Documentation:

• Created docs/SPRINT-3-FEATURES.md
• Complete feature documentation (2,300+ lines)
• Detailed descriptions of all new features
• Technical implementation details
• Usage guides and examples
• Keyboard shortcuts reference
• File structure overview
• Statistics and metrics

Documentation Sections:

1. Overview and introduction
2. Modern Editor Interface
   • CommandBar layout and tools
   • Properties panel features
   • Status bar information
   • Zoom controls
   • Undo/Redo system
3. Overlay Quick Select
   • Keyboard shortcuts
   • Help panel and animations
   • Modern UI elements
   • Configurable appearance
4. Toast Notification System
   • Control types and design
   • Manager functionality
   • Animations and auto-hide
   • Replacement of balloon tips
5. Upload History
   • Window layout and features
   • Actions and functionality
   • Re-upload process
   • Menu integration
6. Keyboard shortcuts reference
7. File structure
8. Technical changes
9. Localization details
10. Usage instructions
11. Statistics

This completes the localization and documentation for Sprint 3. All features are now fully documented and ready for translation.