Add logging improvements, SQLite results store, and enhance sidecar launcher

- **Logging**: Introduced configurable logging across Worker, Hub, and Dashboard with environment-driven settings and logging scopes for contextual logs.
- **SQLite Results Store**: Added a durable SQLite-backed adapter for storing runs, tests, and commands in Hub.
- **Dashboard**: Added connection status banner for enhanced accessibility in MainLayout.
- **Sidecar Launcher**: Replaced raw console outputs with structured logging and added validation for expected JSON payload structure.
- **Build**: Updated `Directory.Build.props` to treat nullable warnings as errors in CI builds.

Author: ereticul

Agenix.PlaywrightGrid.HubClient/Exceptions.cs

@@ -0,0 +1,76 @@
+#region License
+// Copyright (c) 2025 Agenix
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#endregion
+
+namespace Agenix.PlaywrightGrid.HubClient;
+
+/// <summary>
+///     Thrown when the hub indicates that capacity is temporarily unavailable for the requested label.
+///     Typical HTTP status: 503 Service Unavailable.
+/// </summary>
+public sealed class CapacityUnavailableException : Exception
+{
+    public CapacityUnavailableException()
+    {
+    }
+
+    public CapacityUnavailableException(string message) : base(message)
+    {
+    }
+
+    public CapacityUnavailableException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}
+
+/// <summary>
+///     Thrown when the hub rejects the request due to authentication/authorization failure.
+///     Typical HTTP statuses: 401 Unauthorized or 403 Forbidden.
+/// </summary>
+public sealed class AuthenticationException : Exception
+{
+    public AuthenticationException()
+    {
+    }
+
+    public AuthenticationException(string message) : base(message)
+    {
+    }
+
+    public AuthenticationException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}
+
+/// <summary>
+///     Thrown when the hub returns a response that violates the expected protocol contract
+///     (e.g., missing fields, unexpected shape, or unrecognized status codes).
+/// </summary>
+public sealed class ProtocolException : Exception
+{
+    public ProtocolException()
+    {
+    }
+
+    public ProtocolException(string message) : base(message)
+    {
+    }
+
+    public ProtocolException(string message, Exception innerException) : base(message, innerException)
+    {
+    }
+}