feat(ui): Online Inbox settings tab + auth-code/PKCE login

New Settings tab: enable toggle, config fields, sign-in/out + status.
OnlineLoginService runs the PKCE loopback flow (Duende.IdentityModel.OidcClient
7.1.0), opens the system browser, captures the callback, hands the refresh
token to the Worker. en/de localized. Fixes: loopback callback URL built from
host:port base (avoids doubled redirect path); PollIntervalSeconds threaded
through the state DTO so it loads instead of resetting to 60.

Visual layout + the live sign-in round-trip need manual verification.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/ViewModels/Modals/SettingsModalViewModel.cs

@@ -17,6 +17,7 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
     public WorktreesSettingsTabViewModel Worktrees { get; }
     public FilesSettingsTabViewModel Files { get; }
     public PrimeClaudeTabViewModel Prime { get; }
+    public OnlineInboxSettingsViewModel OnlineInbox { get; }
 
     [ObservableProperty] private string _validationError = "";
     [ObservableProperty] private bool _isBusy;
@@ -25,6 +26,7 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
     public Action? CloseAction { get; set; }
 
     public SettingsModalViewModel(IWorkerClient worker, PrimeClaudeTabViewModel prime,
+                                  IOnlineLoginService onlineLoginService,
                                   ILocalizer localizer, AppSettings appSettings)
     {
         _worker = worker;
@@ -36,6 +38,7 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
         Worktrees = new WorktreesSettingsTabViewModel(worker);
         Files = new FilesSettingsTabViewModel(worker);
         Prime = prime;
+        OnlineInbox = new OnlineInboxSettingsViewModel(worker, onlineLoginService);
     }
 
     public async Task LoadAsync()
@@ -65,6 +68,7 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
             else StatusMessage = Loc.T("vm.settingsModal.workerOffline");
 
             await Prime.LoadAsync();
+            await OnlineInbox.LoadAsync();
         }
         finally { IsBusy = false; }
     }