Fix header/subtitle FOUC and Simple Mode translations

- Refactor tool page headers to prevent flash of unstyled content (FOUC):
  - Move long SEO titles to visually hidden `<h1>` tags for search engines.
  - Use `<h2>` for visible tool titles with text matching i18n keys.
  - Sync default HTML subtitles with [tools.json]

src/js/i18n/index.ts

@@ -1,3 +1,15 @@
-export { initI18n, t, changeLanguage, applyTranslations, rewriteLinks, getLanguageFromUrl, supportedLanguages, languageNames } from './i18n';
+export {
+  initI18n,
+  t,
+  changeLanguage,
+  applyTranslations,
+  rewriteLinks,
+  getLanguageFromUrl,
+  supportedLanguages,
+  languageNames,
+} from './i18n';
 export type { SupportedLanguage } from './i18n';
-export { createLanguageSwitcher, injectLanguageSwitcher } from './language-switcher';
+export {
+  createLanguageSwitcher,
+  injectLanguageSwitcher,
+} from './language-switcher';