Add maskable icon with solid background for Windows taskbar

- Created icon-maskable.svg with full background (no transparency)
- Generated icon-512x512-maskable.png with 90% safe zone
- Updated manifest to use dedicated maskable icon
- Fixes white background issue on Windows taskbar/topbar

nuxt4/nuxt.config.ts

@@ -48,7 +48,7 @@ export default defineNuxtConfig({
           purpose: 'any'
         },
         {
-          src: '/icon-512x512.png',
+          src: '/icon-512x512-maskable.png',
           sizes: '512x512',
           type: 'image/png',
           purpose: 'maskable'