ux(ui): beginner-friendly guidance across trainee/admin flows + fix AdminUsers to username

- Chat: add 'How to practice' guide card; render revealed persona as readable
  label:value grid instead of JSON.
- AdminUsers: switch create form to username (matches username login), add step guide,
  role explanation (user=ฝึก, admin=ดูแลระบบ).
- Analytics (admin overview) + MyBoard (personal): add plain-Thai explainer subtitle.
- Rebuild dist. Verified user create with username -> 201.

frontend/dist/assets/Analytics-Pg5dwyd7.css

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+.stat[data-v-45823937]{text-align:center;min-width:110px}.stat div[data-v-45823937]{color:var(--muted);font-size:12px}.stat strong[data-v-45823937]{font-size:20px}.filter-bar[data-v-45823937]{display:flex;align-items:center;gap:10px;flex-wrap:wrap}.filter-bar input[type=date][data-v-45823937]{width:auto}.grid[data-v-45823937]{display:grid;grid-template-columns:repeat(auto-fill,minmax(220px,1fr));gap:14px}.tier-card[data-v-45823937]{display:flex;flex-direction:column}.badge.won[data-v-45823937]{background:#dcfce7;color:#166534}.badge.lost[data-v-45823937]{background:#fee2e2;color:#991b1b}.badge.not_tried[data-v-45823937]{background:#eef2ff;color:#4338ca}.bar[data-v-45823937]{background:#eef0f5;border-radius:999px;height:10px;overflow:hidden;margin-top:10px}.bar-fill[data-v-45823937]{background:linear-gradient(90deg,var(--accent),var(--accent-2));height:100%;border-radius:999px;transition:width .4s ease}.users-btn[data-v-45823937]{background:#0ea5e9;border-color:transparent;color:#fff;font-weight:600}.users-btn[data-v-45823937]:not(:disabled):hover{background:#0284c7;box-shadow:0 6px 16px #0ea5e959}