refactor: address code review findings across all projects

Apply comprehensive fixes from code reviews including:
- Extract shared utilities (SqlFormatHelper, CellValueConverter, DbDestinationBase)
- Add interface abstractions (IAuthenticationService, IDatabaseMigrator, IMisQueryBuilder)
- Implement SecureStore for encrypted secrets storage
- Fix error handling with proper HTTP status codes and logging
- Optimize double enumeration in DevEtlRegistry
- Add DataSync.Dev README for developer onboarding
- Extract filter panel base classes to reduce duplication
- Update code review docs to mark all issues as fixed

NEW/src/JdeScoping.Client/Auth/IAuthStateProvider.cs

@@ -0,0 +1,32 @@
+using JdeScoping.Core.Models.Auth;
+
+namespace JdeScoping.Client.Auth;
+
+/// <summary>
+/// Interface for managing authentication state in the Blazor client.
+/// Extracted from AuthStateProvider for testability.
+/// </summary>
+public interface IAuthStateProvider
+{
+    /// <summary>
+    /// Called after successful login to update auth state and persist user info.
+    /// </summary>
+    /// <param name="user">Authenticated user info from the server.</param>
+    Task MarkUserAsAuthenticated(UserInfoDto user);
+
+    /// <summary>
+    /// Notifies that authentication state has changed, triggering a re-evaluation.
+    /// </summary>
+    void NotifyAuthenticationStateChanged();
+
+    /// <summary>
+    /// Logs out the user by removing cached data and notifying of state change.
+    /// </summary>
+    Task LogoutAsync();
+
+    /// <summary>
+    /// Gets the current username from the cached user info.
+    /// </summary>
+    /// <returns>The username if authenticated, null otherwise.</returns>
+    Task<string?> GetUsernameAsync();
+}