fix(windows): use system titlebar to prevent black screen on startup

The `titleBarStyle: "Overlay"` setting in tauri.conf.json causes black
screen and crash on Windows due to WebView2 compatibility issues.

Add platform-specific config `tauri.windows.conf.json` to override
titleBarStyle to "Visible" on Windows while keeping the overlay style
on macOS for the immersive UI experience.

Fixes black screen issue on Windows for v3.8.x releases.

src-tauri/tauri.windows.conf.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://schema.tauri.app/config/2",
+  "app": {
+    "windows": [
+      {
+        "label": "main",
+        "titleBarStyle": "Visible"
+      }
+    ]
+  }
+}