feat(ui): Log viewer (#28b)

[jaybarden1]

Parent

Sub-issue of #28.

Summary

Implement the in-app log viewer per .claude/rules/onedrive-logviewer.md. Self-contained — no account dependency.

Scope (~4 production files)

• InMemoryLogSink — Serilog sink + ILogEntryProvider; ring buffer (500 entries); PII scrubbing (email, OS username); thread-safe via ConcurrentQueue + Subject<LogEntry>
• ILogEntryProvider — interface: IObservable<LogEntry> EntryAdded + IReadOnlyList<LogEntry> GetSnapshot()
• LogViewerViewModel — subscribes to EntryAdded, loads snapshot on construction, marshals to UI thread via RxApp.MainThreadScheduler; ObservableCollection<LogEntry> Entries; level filter
• LogViewerView.axaml + .axaml.cs — list bound to Entries, level filter dropdown, auto-scroll toggle
• DI registration in startup (InMemoryLogSink as singleton for both ILogEventSink and ILogEntryProvider)

Acceptance criteria

• Live log entries appear in the viewer as they are emitted
• Level filter hides entries below the selected level
• Auto-scroll toggle works
• Email addresses and OS username are scrubbed from displayed entries
• Ring buffer evicts oldest when capacity (500) is exceeded
• dotnet build zero warnings; dotnet test all pass

Depends on

• #28a (settings shell to host the viewer)