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/dist/assets/plus-CAyBY_kw.js

@@ -0,0 +1,6 @@
+import{k as e}from"./index-Dm2VwslH.js";/**
+ * @license lucide-vue-next v1.0.0 - ISC
+ *
+ * This source code is licensed under the ISC license.
+ * See the LICENSE file in the root directory of this source tree.
+ */const a=e("plus",[["path",{d:"M5 12h14",key:"1ays0h"}],["path",{d:"M12 5v14",key:"s699le"}]]);export{a as P};