Release v1.4.15 with deep performance optimizations and crash prevention

- Convert session persistence to async during downloads (prevents 50-200ms UI freezes)
- Avoid unnecessary Buffer.from() copy on Uint8Array chunks (zero-copy when possible)
- Cache effective speed limit for 2s (eliminates Date object creation per chunk)
- Replace O(n) speed event shift() with pointer-based pruning
- Throttle speed event pruning to every 1.5s instead of per chunk
- Optimize refreshPackageStatus to single-loop counting (was 4 separate filter passes)
- Fix global stall watchdog race condition (re-check abort state before aborting)
- Add coalescing for async session saves (prevents write storms)

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "real-debrid-downloader",
-  "version": "1.4.14",
+  "version": "1.4.15",
   "description": "Real-Debrid Downloader Desktop (Electron + React + TypeScript)",
   "main": "build/main/main/main.js",
   "author": "Sucukdeluxe",