feat(i18n + export + guide): locale-aware scenarios/debrief; CSV export; how-to-use page

Backend:
- SCENARIOS localized (th/en): scenario preamble + persona tone adaptation follow the
  trainee's locale; start accepts {locale} and stores it on the session; send/auto-finish
  use it so debrief text (why/coaching/turning points) is in the active language.
- /api/analytics/export returns per-trainee finished-session CSV (admin).

Frontend:
- Scenario picker labels localize by i18n.locale.
- New /guide page (non-IT how-to), linked from Training.
- Analytics: date-filter bar uses line icon + 'Download CSV' button (fetch w/ auth).
Rebuilt dist.

frontend/src/api/index.js

@@ -47,7 +47,7 @@ export const api = {
   listPersonas: (gid) => request('GET', `/api/groups/${gid}/personas`),
   getPersona: (gid, pid) => request('GET', `/api/groups/${gid}/personas/${pid}`),
   updatePersona: (gid, pid, b) => request('PUT', `/api/groups/${gid}/personas/${pid}`, b),
-  chatStart: (gid, pid, scenario) => request('POST', `/api/chat/${gid}/personas/${pid}/chat/start`, scenario || {}),
+  chatStart: (gid, pid, scenario, locale) => request('POST', `/api/chat/${gid}/personas/${pid}/chat/start`, { scenario, locale }),
   chatResume: (gid, pid) => request('GET', `/api/chat/${gid}/personas/${pid}/chat/resume`),
   chatSend: (gid, pid, text) => request('POST', `/api/chat/${gid}/personas/${pid}/chat/send`, { text }),
   chatFinish: (gid, pid) => request('POST', `/api/chat/${gid}/personas/${pid}/chat/finish`),