Add icon.ico and icon.png for Windows and Linux builds

Windows requires icon.ico for the resource file during tauri-build.
Generated from existing 128x128@2x.png placeholder.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src-tauri/tauri.conf.json

@@ -31,7 +31,9 @@
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",
-      "icons/128x128@2x.png"
+      "icons/128x128@2x.png",
+      "icons/icon.ico",
+      "icons/icon.png"
     ]
   },
   "plugins": {}