no-em-dash pre-commit hook for user-facing strings, existing em-dashes rewritten

frontend/scripts/no-em-dash.ts walks src/ with the TypeScript parser and flags
an em-dash in JSX text, string literals and template text. A bare "—"
placeholder passes, separators go in no-em-dash.allow.json. The script is
identical to index's copy.

frontend/scripts/no-em-dash.allow.json

@@ -0,0 +1 @@
+{}

frontend/scripts/no-em-dash.ts

@@ -0,0 +1,77 @@
+/**
+ * Flags em-dashes in user-facing strings: JSX text, string literals and
+ * template literal text under `src/`. Comments are not checked. See Microcopy
+ * in the root DESIGN-GUIDELINES.md.
+ *
+ *     bun frontend/scripts/no-em-dash.ts [files…]   # no files: all of src/
+ *
+ * A literal that is only "—" is the empty-value placeholder and passes. The
+ * other exception, a separator inside one line, goes in `no-em-dash.allow.json`
+ * as the file (relative to `src/`) and the exact string. A template literal is
+ * listed by its source text, backticks included.
+ *
+ * The same file lives in app/ and index/. Keep the two identical.
+ */
+
+import { readdirSync, readFileSync } from "node:fs"
+import { join, relative, resolve } from "node:path"
+import ts from "typescript"
+import allow from "./no-em-dash.allow.json"
+
+const src = resolve(import.meta.dir, "../src")
+const skip = /(^|\/)client\/|\.gen\.ts$|\.(check|spec|test)\.ts$/
+const dash = /—|—/
+
+const allowed = allow as Record<string, string[]>
+const squash = (s: string) => s.replace(/\s+/g, " ").trim()
+
+const walk = (dir: string): string[] =>
+  readdirSync(dir, { withFileTypes: true }).flatMap((e) =>
+    e.isDirectory() ? walk(join(dir, e.name)) : [join(dir, e.name)],
+  )
+
+const args = process.argv.slice(2).map((f) => resolve(f))
+const files = (args.length ? args : walk(src)).filter((f) => {
+  const rel = relative(src, f)
+  return /\.tsx?$/.test(f) && !rel.startsWith("..") && !skip.test(rel)
+})
+
+let found = 0
+for (const file of files) {
+  const rel = relative(src, file)
+  const sf = ts.createSourceFile(
+    file,
+    readFileSync(file, "utf8"),
+    ts.ScriptTarget.Latest,
+    true,
+    file.endsWith(".tsx") ? ts.ScriptKind.TSX : ts.ScriptKind.TS,
+  )
+  const report = (node: ts.Node, text: string) => {
+    if (text === "—" || allowed[rel]?.includes(text)) return
+    const { line } = sf.getLineAndCharacterOfPosition(node.getStart(sf))
+    console.log(`${relative(process.cwd(), file)}:${line + 1}: ${text}`)
+    found++
+  }
+  const visit = (node: ts.Node) => {
+    if (ts.isTemplateExpression(node)) {
+      const parts = [node.head, ...node.templateSpans.map((s) => s.literal)]
+      if (parts.some((p) => dash.test(p.text))) report(node, node.getText(sf))
+    } else if (
+      (ts.isStringLiteral(node) ||
+        ts.isNoSubstitutionTemplateLiteral(node) ||
+        ts.isJsxText(node)) &&
+      dash.test(node.text)
+    ) {
+      report(node, squash(node.text))
+    }
+    ts.forEachChild(node, visit)
+  }
+  visit(sf)
+}
+
+if (found) {
+  console.log(
+    `\n${found} em-dash(es) in user-facing strings. Rewrite them (Microcopy in DESIGN-GUIDELINES.md) or allowlist a separator in frontend/scripts/no-em-dash.allow.json.`,
+  )
+  process.exit(1)
+}