fix(theme): make checkboxes and loading spinner follow the theme accent

Native checkboxes in settings (managed types, base-model skip list) and the
folder-root delete modal had no color declaration, so checked state used the
browser default blue; the loading spinner used hardcoded #3498db/#f3f3f3.
Give the checkboxes accent-color: var(--lora-accent) and derive the spinner
track/arc from the accent token so all themes and presets stay consistent.

static/css/components/modal/settings-modal.css

@@ -607,6 +607,7 @@
 
 .base-model-skip-option input {
     margin: 0;
+    accent-color: var(--lora-accent);
 }
 
 .base-model-skip-option span {
@@ -1849,6 +1850,7 @@ input:checked + .toggle-slider:before {
 
 .other-subtype-checkbox input[type="checkbox"] {
     cursor: pointer;
+    accent-color: var(--lora-accent);
 }
 
 .other-subtype-toggles.is-disabled {