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
