Refactor client settings fields, fix logs overwrite, improve settings modal

- Replace FieldBounds with generic ClientSettingsField<T> abstraction
- Add IClientSettingsField, ClientSettingsMinMaxField, ClientSettingsBoolField
- Put abstractions in Models/Client/Abstractions namespace
- Fields hold values; CoerceValue for clamping on set
- ClientSettingsService hydrates fields on load; ToDto for save
- Settings modal binds directly to field values, collapsible only for All context
- Add LogsPreView with ShouldRender freeze to prevent Blazor overwriting JS-owned logs DOM
- Add left margin to static group heading in single-section mode

src/DnsmasqWebUI/Models/Client/Abstractions/ClientSettingsField.cs

@@ -0,0 +1,33 @@
+namespace DnsmasqWebUI.Models.Client.Abstractions;
+
+/// <summary>
+/// Generic base for client settings fields. Holds the value and provides coercion on set.
+/// </summary>
+public abstract class ClientSettingsField<T> : IClientSettingsField
+{
+    public string DisplayName { get; }
+    private T _value = default!;
+
+    public virtual T Value
+    {
+        get => _value;
+        set => _value = CoerceValue(value);
+    }
+
+    object? IClientSettingsField.Value
+    {
+        get => Value;
+        set => Value = (T)value!;
+    }
+
+    public abstract string? Validate(T value);
+
+    string? IClientSettingsField.Validate() => Validate(Value);
+
+    protected virtual T CoerceValue(T value) => value;
+
+    protected ClientSettingsField(string displayName)
+    {
+        DisplayName = displayName;
+    }
+}

src/DnsmasqWebUI/Models/Client/Abstractions/IClientSettingsField.cs

@@ -0,0 +1,11 @@
+namespace DnsmasqWebUI.Models.Client.Abstractions;
+
+/// <summary>
+/// Non-generic interface for heterogeneous iteration over client settings fields.
+/// </summary>
+public interface IClientSettingsField
+{
+    string DisplayName { get; }
+    object? Value { get; set; }
+    string? Validate();
+}

src/DnsmasqWebUI/Models/Client/ClientSettingsBoolField.cs

@@ -0,0 +1,13 @@
+using DnsmasqWebUI.Models.Client.Abstractions;
+
+namespace DnsmasqWebUI.Models.Client;
+
+/// <summary>
+/// Boolean field. No validation or coercion.
+/// </summary>
+public sealed class ClientSettingsBoolField : ClientSettingsField<bool>
+{
+    public ClientSettingsBoolField(string displayName) : base(displayName) { }
+
+    public override string? Validate(bool value) => null;
+}

src/DnsmasqWebUI/Models/Client/ClientSettingsFields.cs

@@ -1,23 +1,66 @@
+using DnsmasqWebUI.Models.Client.Abstractions;
+
 namespace DnsmasqWebUI.Models.Client;
 
 /// <summary>
-/// Per-field metadata for client settings: bounds, display names for validation errors.
-/// Used by SettingsModal (validation on save) and by pages/components (defensive clamp when loading from storage).
+/// Static field definitions and hydration/serialization helpers.
 /// </summary>
 public static class ClientSettingsFields
 {
-    public sealed record FieldBounds(int Min, int Max, string DisplayName)
+    static ClientSettingsFields()
     {
-        public bool IsValid(int value) => value >= Min && value <= Max;
-        public int Clamp(int value) => Math.Clamp(value, Min, Max);
-        public string? Validate(int value) =>
-            IsValid(value) ? null : $"{DisplayName} must be between {Min} and {Max} seconds. You entered {value}.";
+        HydrateFrom(new ClientSettings());
     }
 
-    public static readonly FieldBounds ServiceStatusPollingInterval = new(5, 300, "Service status polling interval");
-    public static readonly FieldBounds RecentLogsPollingInterval = new(5, 300, "Recent logs polling interval");
-    public static readonly FieldBounds AppLogsPollingInterval = new(5, 300, "App logs polling interval");
-    public static readonly FieldBounds LeasesPollingInterval = new(5, 300, "DHCP leases refresh interval");
-    public static readonly FieldBounds RecentLogsMaxLines = new(100, 2000, "Recent logs max lines");
-    public static readonly FieldBounds AppLogsMaxLines = new(100, 2000, "App logs max lines");
+    public static readonly ClientSettingsMinMaxField ServiceStatusPollingInterval = new("Service status polling interval", 5, 300);
+    public static readonly ClientSettingsMinMaxField RecentLogsPollingInterval = new("Recent logs polling interval", 5, 300);
+    public static readonly ClientSettingsMinMaxField AppLogsPollingInterval = new("App logs polling interval", 5, 300);
+    public static readonly ClientSettingsMinMaxField LeasesPollingInterval = new("DHCP leases refresh interval", 5, 300);
+    public static readonly ClientSettingsMinMaxField RecentLogsMaxLines = new("Recent logs max lines", 100, 2000, "lines");
+    public static readonly ClientSettingsMinMaxField AppLogsMaxLines = new("App logs max lines", 100, 2000, "lines");
+    public static readonly ClientSettingsBoolField RecentLogsAutoScroll = new("Recent logs auto-scroll");
+    public static readonly ClientSettingsBoolField AppLogsAutoScroll = new("App logs auto-scroll");
+
+    private static readonly (string Section, IClientSettingsField Field)[] ValidationMap =
+    [
+        (SettingsModalSections.ServiceStatus, ServiceStatusPollingInterval),
+        (SettingsModalSections.Logs, RecentLogsPollingInterval),
+        (SettingsModalSections.AppLogs, AppLogsPollingInterval),
+        (SettingsModalSections.Leases, LeasesPollingInterval),
+        (SettingsModalSections.RecentLogsDisplay, RecentLogsMaxLines),
+        (SettingsModalSections.RecentLogsDisplay, RecentLogsAutoScroll),
+        (SettingsModalSections.AppLogsDisplay, AppLogsMaxLines),
+        (SettingsModalSections.AppLogsDisplay, AppLogsAutoScroll),
+    ];
+
+    public static IReadOnlyList<IClientSettingsField> All => [ServiceStatusPollingInterval, RecentLogsPollingInterval, AppLogsPollingInterval, LeasesPollingInterval, RecentLogsMaxLines, AppLogsMaxLines, RecentLogsAutoScroll, AppLogsAutoScroll];
+
+    public static IReadOnlyList<(string Section, IClientSettingsField Field)> ValidationChecks => ValidationMap;
+
+    public static void HydrateFrom(ClientSettings dto)
+    {
+        ServiceStatusPollingInterval.Value = dto.ServiceStatusPollingIntervalSeconds;
+        RecentLogsPollingInterval.Value = dto.RecentLogsPollingIntervalSeconds;
+        AppLogsPollingInterval.Value = dto.AppLogsPollingIntervalSeconds;
+        LeasesPollingInterval.Value = dto.LeasesPollingIntervalSeconds;
+        RecentLogsMaxLines.Value = dto.RecentLogsMaxLines;
+        RecentLogsAutoScroll.Value = dto.RecentLogsAutoScroll;
+        AppLogsMaxLines.Value = dto.AppLogsMaxLines;
+        AppLogsAutoScroll.Value = dto.AppLogsAutoScroll;
+    }
+
+    public static ClientSettings ToDto()
+    {
+        return new ClientSettings
+        {
+            ServiceStatusPollingIntervalSeconds = ServiceStatusPollingInterval.Value,
+            RecentLogsPollingIntervalSeconds = RecentLogsPollingInterval.Value,
+            AppLogsPollingIntervalSeconds = AppLogsPollingInterval.Value,
+            LeasesPollingIntervalSeconds = LeasesPollingInterval.Value,
+            RecentLogsMaxLines = RecentLogsMaxLines.Value,
+            RecentLogsAutoScroll = RecentLogsAutoScroll.Value,
+            AppLogsMaxLines = AppLogsMaxLines.Value,
+            AppLogsAutoScroll = AppLogsAutoScroll.Value,
+        };
+    }
 }

src/DnsmasqWebUI/Models/Client/ClientSettingsMinMaxField.cs

@@ -0,0 +1,33 @@
+using DnsmasqWebUI.Models.Client.Abstractions;
+
+namespace DnsmasqWebUI.Models.Client;
+
+/// <summary>
+/// Min/max bounded integer field with optional clamping on set.
+/// </summary>
+public sealed class ClientSettingsMinMaxField : ClientSettingsField<int>
+{
+    public int Min { get; }
+    public int Max { get; }
+    public string Unit { get; }
+    public bool AllowClamp { get; }
+
+    public ClientSettingsMinMaxField(string displayName, int min, int max, string unit = "seconds", bool allowClamp = true)
+        : base(displayName)
+    {
+        Min = min;
+        Max = max;
+        Unit = unit;
+        AllowClamp = allowClamp;
+    }
+
+    public override string? Validate(int value)
+    {
+        if (value < Min || value > Max)
+            return $"{DisplayName} must be between {Min} and {Max} {Unit}. You entered {value}.";
+        return null;
+    }
+
+    protected override int CoerceValue(int value) =>
+        AllowClamp ? Math.Clamp(value, Min, Max) : value;
+}