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.Web/Controllers/Admin/ViewModels/EmailChannelEditViewModel.cs

@@ -0,0 +1,57 @@
+using System.ComponentModel.DataAnnotations;
+using HrynCo.NotificationService.DAL.Abstract.Providers;
+
+namespace HrynCo.NotificationService.Web.Controllers.Admin.ViewModels;
+
+public class EmailChannelEditViewModel
+{
+    public Guid Id { get; set; }
+
+    // ── Channel fields ─────────────────────────────────────────────────
+    [Required]
+    public string ServiceName { get; set; } = "";
+
+    public int Priority { get; set; } = 1;
+
+    public EmailChannelType EmailChannelType { get; set; } = EmailChannelType.Smtp;
+
+    public int? DailyLimit { get; set; }
+
+    public int? MonthlyLimit { get; set; }
+
+    [Range(1, 100)]
+    public int WarnThresholdPercent { get; set; } = 90;
+
+    public bool IsActive { get; set; } = true;
+
+    // ── SMTP Settings ──────────────────────────────────────────────────
+    [Required]
+    public string Host { get; set; } = "";
+
+    [Range(1, 65535)]
+    public int Port { get; set; } = 587;
+
+    [Required]
+    public string Username { get; set; } = "";
+
+    [Required]
+    public string Password { get; set; } = "";
+
+    public bool UseSsl { get; set; } = true;
+
+    [Required, EmailAddress]
+    public string FromEmail { get; set; } = "";
+
+    [Required]
+    public string FromName { get; set; } = "";
+
+    [Required]
+    public string AppDisplayName { get; set; } = "";
+
+    [Required]
+    public string AppBaseUrl { get; set; } = "";
+
+    // ── Computed ───────────────────────────────────────────────────────
+    public bool IsNew => Id == Guid.Empty;
+    public string PageTitle => IsNew ? "Create Channel" : "Edit Channel";
+}