Fix document sync stuck and improve sync progress and offline fallback

[fboucher-os]

Fixes #196

Root Cause

1. Unbounded sequential HTTP round-trips with zero progress: During SyncService.PullAsync, all posts were retrieved individually via apiClient.GetPost(id) even for read posts, with zero progress events emitted during the post pulling stage. On larger or cleared-cache libraries, this resulted in the UI appearing frozen at "Pulling remote changes...".
2. Missing error handling during post retrieval: An individual network or 404 failure during post pulling caused unhandled exceptions that terminated the sync run before SyncHtmlAsync could download offline content, leaving the sync status hanging.
3. UI / Service sync lifecycle misalignment: Posts.razor's sync button and progress bar were controlled by a local disconnected boolean that did not reflect ISyncService.IsSyncing. Firing SyncNow() while a background sync was running would return immediately and reset isSyncing = false, while background progress events continued firing without clear completion signals.
4. Offline reader fallback: OfflineDataService.GetPostHtmlAsync did not attempt an online fetch-and-cache fallback when online and content was not yet cached locally, causing "Content not available" messages in the reader before full sync completed.

Changes Made

1. SyncProgressEventArgs.cs:
   • Added public bool IsComplete { get; } to unambiguously notify subscribers when a synchronization session completes or fails.
2. IDataService.cs & PostNoteClient.cs:
   • Added bool IsSyncing { get; } property to expose sync state to UI components.
3. OfflineDataService.cs:
   • Implemented IsSyncing => syncService.IsSyncing.
   • GetPost(id): wrapped online fetch in try/catch with fallback to localDataService.GetPostAsync(id).
   • GetPostHtmlAsync(postId): if not cached locally and online, fetches from apiClient.GetPostHtmlAsync(postId), caches it locally via localHtmlStorageService.SavePostHtmlAsync, and returns it.
4. SyncService.cs:
   • Thread-safe SyncAsync() task sharing: returns the currently running task when a sync is already active rather than dropping it.
   • Failure reporting: emits SyncProgressChanged(..., isComplete: true) with the error message in the catch block and logs before rethrowing.
   • PullAsync optimizations:
     • Directly creates and saves Post from PostL for read posts without making redundant GetPost(id) HTTP round-trips.
     • Safely falls back to summary metadata if an individual GetPost(id) fails for an unread post.
     • Reports incremental progress (Pulling {current} of {total} posts...) so users receive immediate feedback.
     • Emits completion progress event with isComplete: true.
5. Posts.razor:
   • Disables and shows loading indicator on the Sync button when syncing.
   • Listens to e.IsComplete to reload posts and auto-clears progress status after a short delay.
6. Tests:
   • Added unit tests in SyncServiceTests.cs and PostsTests.cs covering read post bypass, fallback on network error, progress events, task reuse, and completion handling.
   • Verified that all 362 tests across the solution pass with zero errors and no nullable warnings.

[github-actions]

Package	Line Rate	Branch Rate	Health
NoteBookmark.AIServices	97%	64%	✔
NoteBookmark.Api	89%	80%	✔
NoteBookmark.BlazorApp	4%	10%	❌
NoteBookmark.Domain	93%	76%	✔
NoteBookmark.ServiceDefaults	96%	75%	✔
NoteBookmark.SharedUI	22%	27%	❌
Summary	57% (1113 / 1938)	49% (311 / 633)	❌