feat: simple-mode code-block highlighting, prompt, and copy rules

Add a custom Shiki grammar for the simple-mode command language
(src/grammars/rdbms.mjs), registered with Expressive Code. Two language ids
share it: rdbms (real commands) and rdbms-syntax (abstract templates).
Simple-mode blocks now highlight; advanced examples keep sql.

Separation + copy ergonomics via CSS (global.css): a decorative, copy-safe
"> " prompt on rdbms command lines (not in the copy buffer), and the copy
button hidden on multi-command rdbms blocks and on rdbms-syntax templates
(the app input is single-line, so a multi-command paste is not runnable);
single-command, sql, and sh blocks keep copy.

Content: convert 22 simple-mode fences to rdbms; lead the simplest examples
(first project, Tables reference) with bare "with pk" (the beginner default
that creates a ready-made id key), pointing to the named form. Record the
fence + prompt conventions in STYLE.md.

website/astro.config.mjs

@@ -4,6 +4,10 @@ import starlight from '@astrojs/starlight';
 
 import tailwindcss from '@tailwindcss/vite';
 
+// Custom highlight grammar for the playground's simple-mode command language
+// (advanced-mode examples use the built-in `sql` grammar). Fence with ```rdbms.
+import rdbmsLang, { rdbmsSyntax } from './src/grammars/rdbms.mjs';
+
 // https://astro.build/config
 export default defineConfig({
 	// TODO(Phase B/SEO): set `site` to the production URL once the domain is
@@ -21,6 +25,8 @@ export default defineConfig({
 			// public home. Omitted for now rather than linking the wrong repo.
 			// social: [{ icon: 'github', label: 'GitHub', href: '…' }],
 			customCss: ['./src/styles/global.css'],
+			// Register the simple-mode command grammar with Expressive Code (Shiki).
+			expressiveCode: { shiki: { langs: [rdbmsLang, rdbmsSyntax] } },
 			// Pragmatic structure (ADR-0044 §7 / website/STYLE.md): Getting
 			// started, Guides, Reference, Concepts. Autogenerated per directory;
 			// in-section order is controlled by each page's `sidebar.order`