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/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;
+}