🎨 (basic): Organiza estilos de Basic e Intro

Divide `basic.css` en parciales SCSS bajo `assets/scss/basic/`, integra
`normalize.css` en `basic.min.css` y compila `intro.scss` por separado a
`intro.min.css`.

assets/scss/basic/_root.scss

@@ -0,0 +1,27 @@
+:root {
+    // Font families
+    --val-font-sans: system-ui,-apple-system,"Segoe UI",Roboto,"Helvetica Neue",Arial,"Noto Sans","Liberation Sans",sans-serif;
+    --val-font-serif: Georgia,"Times New Roman",serif;
+    --val-font-monospace: SFMono-Regular,Menlo,Monaco,Consolas,"Liberation Mono","Courier New",monospace;
+    --val-font-family: var(--val-font-sans);
+    // Font size
+    --val-fs--base: 1rem;
+    // Font weight
+    --val-fw--base: 400;
+    // Line height
+    --val-lh--base: 1.5;
+    // Core Colors
+    --val-color--primary: #0d6efd;
+    --val-color--secondary: #6c757d;
+    --val-color--success: #198754;
+    --val-color--info: #0dcaf0;
+    --val-color--warning: #ffc107;
+    --val-color--danger: #dc3545;
+    // Other Colors
+    --val-color--bg: #fafafa;
+    --val-color--bg--muted: #dee2e6;
+    --val-color--text: #212529;
+    --val-color--text--muted: color-mix(in srgb, var(--val-color--text) 50%, var(--val-color--bg));
+    --val-color--switch-off: #adb5bd;
+    --val-color--border: #212529;
+}