fix(window): add minWidth/minHeight to Windows platform config

Tauri 2.0 platform config merging is shallow, not deep. The Windows
config only specified titleBarStyle, causing minWidth/minHeight to
be missing on Windows. This allowed users to resize the window below
900px, causing header elements to misalign.

src-tauri/tauri.windows.conf.json

@@ -4,7 +4,9 @@
     "windows": [
       {
         "label": "main",
-        "titleBarStyle": "Visible"
+        "titleBarStyle": "Visible",
+        "minWidth": 900,
+        "minHeight": 600
       }
     ]
   }