Fix app freezes and false provider cooldowns

- Make saveSettings async to stop blocking the event loop during downloads
- Add 120ms minimum gap for forced state emissions to prevent rapid-fire IPC
- Fix circuit breaker feedback loop: reset failure count after cooldown expires
- Add 120s time-decay for failure counter (transient bursts don't snowball)
- Raise circuit breaker threshold from 5 to 8 consecutive failures
- Stop counting network stalls as provider failures
- Items without a provider only check primary provider cooldown, not all

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "real-debrid-downloader",
-  "version": "1.5.18",
+  "version": "1.5.19",
   "description": "Real-Debrid Downloader Desktop (Electron + React + TypeScript)",
   "main": "build/main/main/main.js",
   "author": "Sucukdeluxe",