fix(desktop): enable image drag-and-drop in the Tauri window

Tauri's dragDropEnabled defaults to true, which makes the native OS file
drop intercept and swallow the event so the webview's HTML5 drop handler
(useCanvasDrop) never fires — drag-and-drop of images worked in the browser
but failed in the desktop app. Set dragDropEnabled: false so the webview
receives HTML5 drag/drop natively and the existing web handler works on
desktop too. No web-side code changes.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Danila Poyarkov <dev@dannote.net>

desktop/tauri.conf.json

@@ -14,7 +14,8 @@
       {
         "title": "OpenPencil",
         "width": 1280,
-        "height": 800
+        "height": 800,
+        "dragDropEnabled": false
       }
     ],
     "security": {