Refactor: reorganise namespaces and move service DTOs to Models/Contracts

- Move Client/Http, Services, Parsers under Infrastructure/ (Client, Services, Parsers)
- Move config helpers (DnsmasqConfKeys, DnsmasqOptionTooltips, DnsmasqFileEncoding, EffectiveConfigFieldBuilder) to Infrastructure/Helpers/Config; ApiJsonOptions to Helpers/Http
- Move Configuration types (ApplicationOptions, DnsmasqOptions, DnsmasqOptionsValidator) to Models/Config
- Merge Models/Status and Models/EffectiveConfig into Models/Dnsmasq and Models/Dnsmasq/EffectiveConfig
- Move Client/Models and Client/Services to Models/Client and Infrastructure/Services
- Move ConfigSetSnapshot, HostsSnapshot, ProcessRunResult, ManagedConfigContent from Infrastructure/Services/Abstractions to Models/Contracts
- Update all namespaces and usings; tests pass

src/DnsmasqWebUI/Models/Client/ClientSettings.cs

@@ -0,0 +1,16 @@
+namespace DnsmasqWebUI.Models.Client;
+
+/// <summary>
+/// Client-side preferences persisted in browser local storage.
+/// </summary>
+public class ClientSettings
+{
+    /// <summary>Polling interval for the service status block (seconds).</summary>
+    public int ServiceStatusPollingIntervalSeconds { get; set; } = 15;
+
+    /// <summary>Polling interval for the recent logs block (seconds).</summary>
+    public int RecentLogsPollingIntervalSeconds { get; set; } = 5;
+
+    /// <summary>Polling interval for the DHCP leases list (seconds).</summary>
+    public int LeasesPollingIntervalSeconds { get; set; } = 5;
+}

src/DnsmasqWebUI/Models/Client/SettingsModalContext.cs

@@ -0,0 +1,19 @@
+namespace DnsmasqWebUI.Models.Client;
+
+/// <summary>
+/// Which subset of client settings to show in the settings modal.
+/// </summary>
+public enum SettingsModalContext
+{
+    /// <summary>Show all settings with optional search.</summary>
+    All,
+
+    /// <summary>Show only service status polling interval.</summary>
+    ServicePolling,
+
+    /// <summary>Show only recent logs polling interval.</summary>
+    LogsPolling,
+
+    /// <summary>Show only DHCP leases refresh interval.</summary>
+    LeasesPolling
+}