fix: mark dmg-license as optional dependency for cross-platform builds

dmg-license is a macOS-only package used for building DMG installers.
Moving it from devDependencies to optionalDependencies allows npm ci
to succeed on Linux and Windows without failing on platform checks.

macOS developers will still get the package when available.
Linux/Windows developers can now run npm ci without errors.

Fixes: npm ci failing on Linux with "EBADPLATFORM" error

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

package.json

@@ -67,10 +67,12 @@
     "tree-kill": "1.2.2"
   },
   "devDependencies": {
-    "dmg-license": "^1.0.11",
     "husky": "9.1.7",
     "lint-staged": "16.2.7",
     "prettier": "3.7.4",
     "vitest": "4.0.16"
+  },
+  "optionalDependencies": {
+    "dmg-license": "^1.0.11"
   }
 }