Fix prefetch race condition, transaction observable leak, and media store getUrl crash

[Copilot]

Three concurrency and error-handling bugs in core shared services causing non-deterministic progress reporting, potential observable memory leaks, and silent media display failures.

Changes

• Prefetch race condition (dia-backend-asset-prefetching.service.ts): Replace Promise.all + concurrent .map with a sequential for...of loop. Concurrent async callbacks were racing on the shared currentCount variable, causing skipped or out-of-order progress callbacks.

  // Before – concurrent writes to currentCount
  await Promise.all(diaBackendAssets.map(async asset => {
    await this.downloadingService.storeRemoteCapture(...);
    currentCount += 1; // race condition
    onStored(currentCount, totalCount);
  }));
  
  // After – sequential, deterministic
  for (const diaBackendAsset of diaBackendAssets) {
    await this.downloadingService.storeRemoteCapture(...);
    currentCount += 1;
    onStored(currentCount, totalCount);
  }

• Transaction observable leak (dia-backend-transaction-repository.service.ts): Add catchError returning EMPTY at the end of the downloadExpired$ pipeline. A failure anywhere in the concatMap chain previously terminated the observable uncleanly, leaving subscribers hanging. Imports EMPTY and catchError from rxjs.

• Media store getUrl crash (media-store.service.ts): Wrap getUrl() body in try/catch. Failures from Capacitor.convertFileSrc(), getUri(), or readWithFileSystem() were propagating as unhandled rejections and silently breaking the display pipeline. Now logs the error and returns '' as a graceful fallback.