fix: updater progress jumping and click-twice issues

- Guard installUpdate() with isDownloading flag to prevent concurrent downloads
- Deduplicate checkForUpdates() with checkInProgress promise
- Track monotonic progress (peakPercent never decreases)
- Reduce startup check delay from 5s to 1s
- Bump desktop to 0.2.4

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

cc-haha/desktop/package.json

@@ -1,7 +1,7 @@
 {
   "name": "heicode-desktop",
   "private": true,
-  "version": "0.2.3",
+  "version": "0.2.4",
   "type": "module",
   "scripts": {
     "dev": "vite",