FRE-606: Add icon generation and tests for Tauri desktop

- Add generate-icons.sh script for cross-platform icon generation
- Generate all required icon sizes (32x32, 128x128, 256x256, 512x512)
- Create Windows .ico and macOS .icns icons
- Add Rust unit tests for core functionality
- Add tauri:test and tauri:icons npm scripts

New files:
- src-tauri/generate-icons.sh
- src-tauri/icons/32x32.png
- src-tauri/icons/128x128.png
- src-tauri/icons/128x128@2x.png
- src-tauri/icons/512x512.png
- src-tauri/icons/icon.ico
- src-tauri/icons/icon.icns
- src-tauri/icons/src/app-icon.svg
- src-tauri/icons/tray-icon.png
- src-tauri/src/tests.rs

Modified:
- src-tauri/src/lib.rs (added tests module)

src-tauri/src/lib.rs

@@ -1,6 +1,8 @@
 pub mod menu;
 pub mod tray;
 pub mod updater;
+#[cfg(test)]
+pub mod tests;
 
 use serde::{Deserialize, Serialize};