Show sync progress bar when downloading offline post text

[fboucher-os]

Feature Description

Add a visual progress indicator (such as a progress bar or status text e.g., "Downloading 3 of 5 posts") during the synchronization process so users have clear feedback when offline text versions of posts are being downloaded and saved locally.

Context

Extracted from feedback in #173:

"One feature that could be very interesting to have the feeling that something is happening during the sync is have some kind of progress bar that shows us that the sync process is downloading the text version to save them locally. Poor percentage or like a progress bar saying like three done of five or something like that"

Proposed Design & Scope

• Surface sync progress in SyncService (e.g., total posts to download, current progress index).
• Expose sync progress event/state to the UI (Posts.razor or global sync notification banner).
• Render a progress bar or status message during active HTML download phases.

[github-actions]

This was generated by AI during triage.

Agent Brief

Category: type:feature

Summary: Add a visual progress indicator during the synchronization process so users have feedback while offline text versions of posts are being downloaded and saved locally.

Current behavior: Sync runs with no visual feedback; the user cannot tell how much of the HTML/offline-text download phase remains (or whether it is progressing at all).

Desired behavior: While the offline text download phase of sync is active, the UI shows a progress indicator — either a progress bar or a status message such as "Downloading 3 of 5 posts" — that updates as posts are processed.

Key interfaces: The sync service (expose progress state such as total posts to download and current index during the HTML download phase) and the UI layer that renders sync status (e.g., the posts page or the global sync notification/banner). Render the indicator while the download phase is active.

Acceptance criteria:

• A progress indicator is visible during the offline-text download phase of sync.
• The indicator reflects the number of posts downloaded relative to the total (e.g., "3 of 5" or equivalent percentage).
• The indicator updates as posts are processed and clears when sync completes (success or failure).
• Existing sync behavior and saved offline content are unaffected.

Out of scope: Progress for other sync phases (e.g., fetching metadata, uploading), changes to the sync logic itself, and any changes to the offline-text format.