chore(frontend): engineering baseline per guide ch.8

- toolchain: vitest+jsdom (15 tests green: permission pure functions
  exhaustive, storage migration), prettier (printWidth 120 to fit the
  dense existing style + 700-line page budget), commitlint conventional
- npm scripts: typecheck / format / test / lint:fix; build fingerprint
  __APP_VERSION__ (version+git hash+build time) shown in Settings > about
- git hooks via core.hooksPath=frontend/.husky: pre-commit lint-staged
  (prettier + eslint --fix best-effort), commit-msg commitlint
- permission pure functions extracted to lib/permissions.ts (single
  source consumed by useAuth/useLogin); AuditLogsPage filters on URL
- backend: resp package unit tests; health version constant aligned
  with frontend package.json

frontend/.prettierrc

@@ -2,6 +2,7 @@
   "singleQuote": true,
   "semi": true,
   "trailingComma": "all",
-  "printWidth": 110,
-  "tabWidth": 2
+  "printWidth": 120,
+  "tabWidth": 2,
+  "_comment": "printWidth 120 贴合本仓既有长行风格;行数预算(700/页)按此口径执行"
 }