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 @@
+{}