ci: add cross-platform builds for Linux, macOS (Intel + ARM), and Windows

- Add macOS Intel build (macos-13 / x86_64-apple-darwin) to CI matrix
- Add artifact upload step to CI build job
- Add release workflow triggered by v* tags with draft GitHub Release
- Add AppImage to Linux bundle targets

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

src-tauri/tauri.conf.json

@@ -27,7 +27,7 @@
   },
   "bundle": {
     "active": true,
-    "targets": ["deb", "rpm", "dmg", "nsis"],
+    "targets": ["deb", "rpm", "appimage", "dmg", "nsis"],
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",