feat: add window state persistence

[vishesh-sachan]

Summary

Adds automatic window state persistence to remember window size and position across app restarts.

Type of Change

• [x] feat: New feature

Related Issue

Closes #878

Changes Made

• Added tauri-plugin-window-state v2.4.0 dependency
• Registered window-state plugin in Rust backend
• Window size, position, and state (maximized/minimized) now persist automatically

The plugin uses Tauri v2's default permission system - all window-state operations are enabled by default with no additional configuration needed. State is saved automatically on window changes and restored on app launch.

Testing

1. Resize/move the window
2. Close the app
3. Reopen - window should restore to previous size and position

Desktop App Testing

• [x] Tested on macOS
• [ ] Tested on Windows
• [ ] Tested on Linux

[Leftium]

I often intentionally close Whispering and re-open it to get it back to the the "default" size. So a way to do this would be appreciated. Perhaps another button like the "minimize" button.