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.

.pre-commit-config.yaml

@@ -40,6 +40,16 @@ repos:
         language: pygrep
         files: ^frontend/src/.*\.tsx$
 
+      # No em-dashes in user-facing strings; see Microcopy in the root
+      # DESIGN-GUIDELINES.md. A separator inside one line goes in
+      # frontend/scripts/no-em-dash.allow.json. Whole tree:
+      # `pre-commit run no-em-dash --all-files`.
+      - id: no-em-dash
+        name: no em-dashes in user-facing strings
+        entry: bun frontend/scripts/no-em-dash.ts
+        language: system
+        files: ^frontend/src/.*\.tsx?$
+
       - id: local-ruff-check
         name: ruff check
         entry: uv run ruff check --force-exclude --fix --exit-non-zero-on-fix