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.DAL.EF/Repositories/EmailChannelRepository.cs

@@ -13,6 +13,16 @@ internal sealed class EmailChannelRepository : EfRepository<EmailChannelEntity>,
     {
     }
 
+    public async Task<IReadOnlyList<EmailChannel>> GetAllAsync(CancellationToken ct = default)
+    {
+        var entities = await DbSet
+            .OrderBy(x => x.ServiceName)
+            .ThenBy(x => x.Priority)
+            .ToListAsync(ct);
+
+        return entities.Select(MapToDomain).ToList();
+    }
+
     public async Task<IReadOnlyList<EmailChannel>> GetByServiceAsync(string serviceName, CancellationToken ct = default)
     {
         var entities = await DbSet