Separate pause/resume buttons, fix hybrid extraction during pause

- Pause button is now one-way (orange glow when paused, disabled when
  already paused). Start button resumes from pause.
- Fix hybrid extraction attempting incomplete multi-part archives when
  paused: disk-fallback now blocks any non-terminal item status, not
  just downloading/validating/integrity_check.

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

src/main/download-manager.ts

@@ -4786,7 +4786,7 @@ export class DownloadManager extends EventEmitter {
       }
       const anyActivelyProcessing = missingParts.some((part) => {
         const status = pendingItemStatus.get(pathKey(part));
-        return status === "downloading" || status === "validating" || status === "integrity_check";
+        return status !== undefined && status !== "failed" && status !== "cancelled";
       });
       if (anyActivelyProcessing) {
         continue;