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/Components/Modals/SettingsModal.razor.css

@@ -86,6 +86,10 @@
     transform: rotate(135deg);
 }
 
+.settings-group-title-static {
+    margin-left: 0.75rem;
+}
+
 .settings-group-content {
     max-height: 0;
     overflow: hidden;