feat(ui): add custom theme color and polish light-mode theming

Support a custom accent theme across nav, terminal, scrollbars, and loaders.
Improve light-mode tokens, helper popup placement, and theme persistence
(including purple→custom migration). Cover with appearance and scrollbar tests.

tests/Feature/LightModeScrollbarTest.php

@@ -0,0 +1,11 @@
+<?php
+
+test('native scrollbars follow the active theme', function () {
+    $styles = file_get_contents(resource_path('css/app.css'));
+
+    expect($styles)
+        ->toContain('html {')
+        ->toContain('color-scheme: light;')
+        ->toContain('html.dark {')
+        ->toContain('color-scheme: dark;');
+});