Refactor settings modal to use ISettingsModalService and SettingsModalSections

- Replace SettingsModalContext with SettingsModalSections
- Add SettingsModalService for modal state management
- Add StatusSection.razor.css for status-dot styles
- Update layout, NavMenu, and modal components
- Update tests for config/settings changes

src/DnsmasqWebUI/Infrastructure/Services/Abstractions/ISettingsModalService.cs

@@ -0,0 +1,23 @@
+using DnsmasqWebUI.Models.Client;
+
+namespace DnsmasqWebUI.Infrastructure.Services.Abstractions;
+
+/// <summary>
+/// Scoped service to open the app-level settings modal from anywhere (e.g. NavMenu, pages).
+/// MainLayout subscribes to <see cref="OpenRequested"/> and hosts the modal.
+/// Subscribe to <see cref="SettingsChanged"/> to refresh when settings are saved/closed.
+/// </summary>
+public interface ISettingsModalService
+{
+    /// <summary>Raised when <see cref="Open"/> is called. MainLayout subscribes and shows the modal.</summary>
+    event Action<SettingsModalContext, string>? OpenRequested;
+
+    /// <summary>Raised when the modal is closed (save or cancel). Pages subscribe to reload their intervals.</summary>
+    event Action? SettingsChanged;
+
+    /// <summary>Opens the settings modal with the given context and title.</summary>
+    void Open(SettingsModalContext context, string title);
+
+    /// <summary>Called when the modal closes. Raises <see cref="SettingsChanged"/>.</summary>
+    void NotifySettingsChanged();
+}

src/DnsmasqWebUI/Infrastructure/Services/SettingsModalService.cs

@@ -0,0 +1,15 @@
+using DnsmasqWebUI.Infrastructure.Services.Abstractions;
+using DnsmasqWebUI.Models.Client;
+
+namespace DnsmasqWebUI.Infrastructure.Services;
+
+/// <inheritdoc cref="ISettingsModalService" />
+public sealed class SettingsModalService : ISettingsModalService
+{
+    public event Action<SettingsModalContext, string>? OpenRequested;
+    public event Action? SettingsChanged;
+
+    public void Open(SettingsModalContext context, string title) => OpenRequested?.Invoke(context, title);
+
+    public void NotifySettingsChanged() => SettingsChanged?.Invoke();
+}