fix: replace placeholder purple square icons with actual Backspace logo

Regenerate all web icons (favicon, PWA, apple-touch) from master 1024x1024
icon.png. Add BrowserWindow icon property for Windows/Linux taskbar icon
in Electron dev mode.

packages/desktop/src/main.ts

@@ -185,6 +185,7 @@ function createWindow(): void {
     minWidth: 940,
     minHeight: 500,
     title: 'Backspace',
+    icon: path.join(__dirname, '..', 'build', 'icon.png'),
     titleBarStyle: process.platform === 'darwin' ? 'hiddenInset' : 'default',
     backgroundColor: '#313338',
     show: false,