feat: polished admin UI styles + email channels admin CRUD

- Extract inline styles to wwwroot/css/admin.css
- Bootstrap Icons for nav and buttons
- Styled page headers, table, empty state, readonly fields
- Email Channels admin: list, create, edit, delete
- GetAllEmailChannelsQuery + handler
- AdminChannelsController with full CRUD
- form id + form= attribute pattern for EditorLayout footer buttons

Ref: IT-628

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

HrynCo.NotificationService.Services/EmailChannels/GetAll/GetAllEmailChannelsHandler.cs

@@ -0,0 +1,30 @@
+using HrynCo.NotificationService.DAL.Abstract;
+using HrynCo.NotificationService.DAL.Abstract.Providers;
+using HrynCo.NotificationService.DAL.Abstract.Repositories;
+using HrynCo.NotificationService.Services.Core;
+using HrynCo.NotificationService.Services.Logging;
+using static HrynCo.NotificationService.Services.Core.ServiceResultHelper;
+
+namespace HrynCo.NotificationService.Services.EmailChannels.GetAll;
+
+internal sealed class GetAllEmailChannelsHandler
+    : RequestHandler<GetAllEmailChannelsQuery, ServiceResult<IReadOnlyList<EmailChannel>>>
+{
+    private readonly IEmailChannelRepository _channels;
+
+    public GetAllEmailChannelsHandler(
+        IContextualSerilogLogger<GetAllEmailChannelsQuery> logger,
+        IUnitOfWork unitOfWork,
+        IEmailChannelRepository channels)
+        : base(logger, unitOfWork)
+    {
+        _channels = channels;
+    }
+
+    protected override async Task<ServiceResult<IReadOnlyList<EmailChannel>>> DoOnHandle(
+        GetAllEmailChannelsQuery request, CancellationToken cancellationToken)
+    {
+        var channels = await _channels.GetAllAsync(cancellationToken);
+        return Success(channels);
+    }
+}