docs: Update CHANGELOG.md to record changes in version 1.1.0

Add support for single instance applications and the ability to manually sign pages
Remove the tray notification balloon and improve the signature page UI
Clean up unused using statements and optimize stroke smoothness

Author: sunquakes

CHANGELOG.md

@@ -4,6 +4,33 @@ All notable changes to this project will be documented in this file.
 
 ---
 
+## [1.1.0] - 2025-03-12
+
+### Added
+- Single instance application support
+  - Prevent multiple instances from running simultaneously
+  - Automatically activate existing window when attempting to launch again
+  - Works correctly even when minimized to system tray
+- Handwritten signature page
+  - Convert signature to Base64 string
+  - Adjustable pen size with smooth strokes
+  - Save signature as image file (PNG, JPEG, BMP, GIF)
+
+### Changed
+- Removed balloon tip notification when minimizing to tray
+- Signature page UI improvements
+  - Buttons changed to icon style
+  - Pen size control repositioned to bottom toolbar
+  - Default pen size changed to 7
+  - Removed image preview, showing Base64 string directly
+- Home page cards repositioned for better visual centering
+
+### Improved
+- Cleaned up unused using statements in MainWindow.xaml.cs
+- Better stroke smoothing with FitToCurve enabled
+
+---
+
 ## [1.0.2] - 2025-03-11
 
 ### Added