Add backup encryption (AES-256-GCM) and directory existence check

- Encrypt sensitive credentials (tokens, passwords) in backup exports
  using AES-256-GCM with PBKDF2 key derivation from OS username
- Backup format v2 with backwards-compatible v1 import
- Show dialog to create non-existent directories when changing
  outputDir, extractDir, or mkvLibraryDir settings

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "real-debrid-downloader",
-  "version": "1.5.49",
+  "version": "1.5.50",
   "description": "Real-Debrid Downloader Desktop (Electron + React + TypeScript)",
   "main": "build/main/main/main.js",
   "author": "Sucukdeluxe",