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/src/grammars/rdbms.mjs

@@ -0,0 +1,76 @@
+// Minimal Shiki / TextMate grammar for the RDBMS Playground *simple-mode*
+// command language, so docs code blocks fenced as ```rdbms get syntax
+// highlighting. Advanced-mode examples use the built-in `sql` grammar.
+//
+// Keyword/clause vocabulary is taken from the in-app help/usage strings
+// (`src/friendly/strings/en-US.yaml`) and the command grammar. Matching is
+// case-insensitive (keywords are case-insensitive in the app; identifiers
+// are case-preserving and intentionally left unscoped so they read as plain
+// text against the coloured keywords).
+//
+// Two language ids share one grammar:
+//   - `rdbms`        — real commands; gets a decorative `> ` prompt via CSS.
+//   - `rdbms-syntax` — abstract syntax templates (with <placeholders>); same
+//                      highlighting, but a distinct `data-language` so the
+//                      prompt (scoped to data-language='rdbms') is NOT added.
+
+const patterns = [
+	{ include: '#comment' },
+	{ include: '#string' },
+	{ include: '#flag' },
+	{ include: '#number' },
+	{ include: '#constant' },
+	{ include: '#type' },
+	{ include: '#keyword' },
+];
+
+const repository = {
+	comment: {
+		match: '#.*$',
+		name: 'comment.line.number-sign.rdbms',
+	},
+	string: {
+		name: 'string.quoted.single.rdbms',
+		begin: "'",
+		end: "'",
+		patterns: [{ match: "''", name: 'constant.character.escape.rdbms' }],
+	},
+	flag: {
+		match: '--[A-Za-z][A-Za-z-]*',
+		name: 'keyword.operator.flag.rdbms',
+	},
+	number: {
+		match: '\\b[0-9]+(?:\\.[0-9]+)?\\b',
+		name: 'constant.numeric.rdbms',
+	},
+	constant: {
+		match: '(?i)\\b(true|false|null)\\b',
+		name: 'constant.language.rdbms',
+	},
+	type: {
+		match: '(?i)\\b(text|int|real|decimal|bool|date|datetime|blob|serial|shortid)\\b',
+		name: 'support.type.rdbms',
+	},
+	keyword: {
+		match:
+			'(?i)\\b(create|table|tables|drop|add|column|with|pk|to|from|into|values|insert|update|set|where|delete|show|data|rename|change|alter|relationship|relationships|index|indexes|on|as|references|constraint|not|unique|default|check|primary|key|cascade|restrict|and|or|in|between|like|is|explain|replay|undo|redo|save|new|load|rebuild|export|import|copy|mode|help|hint|quit|messages|all|last|types|simple|advanced)\\b',
+		name: 'keyword.control.rdbms',
+	},
+};
+
+/** Real simple-mode commands — gets the `> ` prompt (via CSS). */
+export default {
+	name: 'rdbms',
+	scopeName: 'source.rdbms',
+	aliases: ['rdbms-playground'],
+	patterns,
+	repository,
+};
+
+/** Abstract syntax templates — same highlighting, no prompt. */
+export const rdbmsSyntax = {
+	name: 'rdbms-syntax',
+	scopeName: 'source.rdbms-syntax',
+	patterns,
+	repository,
+};