Fix update loop: read APP_VERSION from package.json v1.3.1

APP_VERSION was hardcoded as "1.1.29" in constants.ts causing the app
to always report the old version and re-trigger the update prompt.
Now reads version dynamically from package.json via import.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "real-debrid-downloader",
-  "version": "1.3.0",
+  "version": "1.3.1",
   "description": "Real-Debrid Downloader Desktop (Electron + React + TypeScript)",
   "main": "build/main/main/main.js",
   "author": "Sucukdeluxe",