feat(app): remember window size/position across restarts (window-state plugin)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src-tauri/capabilities/default.json

@@ -11,6 +11,7 @@
     "core:resources:default",
     "core:menu:default",
     "core:tray:default",
-    "notification:default"
+    "notification:default",
+    "window-state:default"
   ]
 }