chore: enforce i18n key coverage via npm test

coverage-check.mjs existed but nothing ran it automatically.
Wired in as a pretest hook so a coverage regression now fails
the test run instead of silently passing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013PphXq1s43DNRj1uWKGXof

frontend/package.json

@@ -6,6 +6,7 @@
   "scripts": {
     "dev": "vite",
     "build": "tsc -b && vite build",
+    "pretest": "node src/i18n/coverage-check.mjs",
     "test": "vitest run"
   },
   "dependencies": {