feat(window): center window on screen by default

Add `center: true` to main window configuration to improve initial
window positioning on Windows and other platforms. This addresses
user feedback about the window appearing in the top-left corner.

src-tauri/tauri.conf.json

@@ -19,7 +19,8 @@
         "minWidth": 800,
         "minHeight": 600,
         "resizable": true,
-        "fullscreen": false
+        "fullscreen": false,
+        "center": true
       }
     ],
     "security": {