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/Services/Interfaces/IOnlineLoginService.cs

@@ -0,0 +1,10 @@
+namespace ClaudeDo.Ui.Services;
+
+public sealed record OnlineLoginResult(bool Success, string? RefreshToken, string? Error);
+
+public interface IOnlineLoginService
+{
+    Task<OnlineLoginResult> LoginAsync(
+        string authority, string clientId, string scope, string redirectUri,
+        CancellationToken ct = default);
+}