feat(i18n): add Thai (th-TH) language support

Frontend:
- Add frontend/src/i18n/th-TH.json (913 keys, full parity with zh-CN)
- Register th-TH in i18n entry with native element-plus-secondary Thai locale
- Add Thai option to language selectors, legacy th_TH alias mapping,
  and browser locale detection (th* -> th-TH)
- Add common.thai label to all existing locale files

Backend:
- Add backend/locales/th-TH.json (177 keys)
- Accept th-TH in language whitelist, schema regex, and MCP question
- Map th* to Thai language name in LLM prompt language selection
- Apply question language to guess-question prompt template

frontend/src/utils/utils.ts

@@ -52,6 +52,9 @@ export const getBrowserLocale = () => {
     const temp = language.toLowerCase().replace('_', '-')
     return temp === 'zh' ? 'zh-CN' : temp === 'zh-cn' ? 'zh-CN' : 'zh-TW'
   }
+  if (language.toLowerCase().startsWith('th')) {
+    return 'th-TH'
+  }
   return language
 }
 export const getLocale = () => {