feat(ui): Per-account settings — sync path, worker count, sync interval (#28d)

[jaybarden1]

Parent

Sub-issue of #28.

Summary

Add per-account settings panel shown when an account is selected in the settings overlay. Covers local sync path, worker count, and sync interval.

Scope (~4 production files)

• AccountSettingsViewModel — ReactiveObject; LocalSyncPath (editable, defaults per onedrive-onboarding.md), WorkerCount (1–10, default 8), SyncInterval (15 min / 30 min / 60 min / 2 h / 4 h / manual); save command persists via IAccountRepository; folder-picker command opens native OS dialog
• AccountSettingsView.axaml + .axaml.cs — path text field + folder-picker button, worker count numeric stepper, sync interval dropdown
• SettingsViewModel — extend to expose selected account and AccountSettingsViewModel instance; bind to WorkspaceViewModel.SelectedAccount
• ISyncScheduler — SetInterval call when sync interval changes

Acceptance criteria

• Local sync path is editable; folder-picker opens OS dialog and populates the field
• Worker count stepper clamps to 1–10
• Sync interval dropdown lists all five options; selection updates the scheduler immediately
• All changes persist across restarts
• dotnet build zero warnings; dotnet test all pass

Depends on

• #28a (settings shell)
• Persistence issue (account entity must be persistable)