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/WorkerClient.cs

@@ -588,7 +588,8 @@ public sealed record OnlineInboxStateDto(
     string ClientId,
     string Scopes,
     string RedirectUri,
-    bool SignedIn);
+    bool SignedIn,
+    int PollIntervalSeconds);
 
 public sealed record OnlineInboxConfigInputDto(
     bool Enabled,