feat(worker,ui): Online Inbox config + auth hub plumbing (Phase 2)

Hub: GetOnlineInboxState / SetOnlineInboxConfig / SetOnlineInboxAuth /
ClearOnlineInboxAuth. WorkerConfig.SaveOnlineInbox persists only the
online_inbox section. OnlineTokenStore + config registered always so hub
methods work when sync is disabled. IWorkerClient surface + all test fakes
synced. RedirectUri config (default http://localhost:8765/callback).

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

src/ClaudeDo.Worker/Config/WorkerConfig.cs

@@ -1,4 +1,5 @@
 using System.Text.Json;
+using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
 using ClaudeDo.Data;
 using ClaudeDo.Worker.Online;
@@ -74,9 +75,38 @@ public sealed class WorkerConfig
         return cfg;
     }
 
+    /// <summary>
+    /// Persists ONLY the <c>online_inbox</c> section back to <paramref name="path"/>
+    /// (defaults to <see cref="DefaultConfigPath"/>) without rewriting any other fields.
+    /// Reads the existing JSON, replaces the <c>online_inbox</c> node, and writes back indented.
+    /// </summary>
+    public void SaveOnlineInbox(string? path = null)
+    {
+        path ??= DefaultConfigPath;
+
+        var root = File.Exists(path)
+            ? JsonNode.Parse(File.ReadAllText(path)) as JsonObject ?? new JsonObject()
+            : new JsonObject();
+
+        root["online_inbox"] = JsonSerializer.SerializeToNode(OnlineInbox, InboxSerializerOpts);
+
+        Directory.CreateDirectory(Path.GetDirectoryName(path)!);
+        File.WriteAllText(path, root.ToJsonString(WriteOpts));
+    }
+
     private static readonly JsonSerializerOptions JsonOpts = new()
     {
         ReadCommentHandling = JsonCommentHandling.Skip,
         AllowTrailingCommas = true,
     };
+
+    private static readonly JsonSerializerOptions InboxSerializerOpts = new()
+    {
+        WriteIndented = false,
+    };
+
+    private static readonly JsonSerializerOptions WriteOpts = new()
+    {
+        WriteIndented = true,
+    };
 }