feat: add release build script

- scripts/build-release.sh: builds, signs, and copies .app to dist/release/
- Bundle targets narrowed to app + dmg
- npm run release convenience script
- README documents the release build workflow

Co-authored-by: Cursor <cursoragent@cursor.com>

src-tauri/tauri.conf.json

@@ -34,7 +34,7 @@
   },
   "bundle": {
     "active": true,
-    "targets": "all",
+    "targets": ["app", "dmg"],
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",