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/Analytics-CQgwlaXy.css

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+.stat[data-v-6eb0872b]{text-align:center;min-width:110px}.stat div[data-v-6eb0872b]{color:var(--muted);font-size:12px}.stat strong[data-v-6eb0872b]{font-size:20px}.filter-bar[data-v-6eb0872b]{display:flex;align-items:center;gap:10px;flex-wrap:wrap}.filter-bar input[type=date][data-v-6eb0872b]{width:auto}.grid[data-v-6eb0872b]{display:grid;grid-template-columns:repeat(auto-fill,minmax(220px,1fr));gap:14px}.tier-card[data-v-6eb0872b]{display:flex;flex-direction:column}.badge.won[data-v-6eb0872b]{background:#dcfce7;color:#166534}.badge.lost[data-v-6eb0872b]{background:#fee2e2;color:#991b1b}.badge.not_tried[data-v-6eb0872b]{background:#eef2ff;color:#4338ca}.bar[data-v-6eb0872b]{background:#eef0f5;border-radius:999px;height:10px;overflow:hidden;margin-top:10px}.bar-fill[data-v-6eb0872b]{background:linear-gradient(90deg,var(--accent),var(--accent-2));height:100%;border-radius:999px;transition:width .4s ease}.users-btn[data-v-6eb0872b]{display:inline-flex;align-items:center;gap:6px;background:#0ea5e9;border-color:transparent;color:#fff;font-weight:600}.users-btn[data-v-6eb0872b]:not(:disabled):hover{background:#0284c7;box-shadow:0 6px 16px #0ea5e959}