feat(personas): 'create persona from this persona' (variant) — practice the same challenge, new identity

Each persona is one-shot (chat once = win/lose locked). To keep training repeatable:
- New endpoint POST /api/groups/<gid>/personas/<pid>/variant creates a NEW persona that is
  a fresh incarnation of the source: LOCKS pain points, objections, negotiation levers,
  tolerance, special/recontact, goal, budget, difficulty, tier, product_context — but VARYS
  name/profession/age/location/background/personality/opener so it isn't an identical copy.
- Added to the same group as a distinct persona (fresh not_tried, so chat-able again).
- UI: on the Personas page, a finished (won/lost) persona gets a
  'สร้างบุคคลต้นแบบจากต้นแบบนี้' button; reload shows the variant.
All 10 backend suites pass. Rebuilt dist.

frontend/dist/assets/Personas-CmLjB15L.css

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+.grid[data-v-20adbc50]{display:grid;grid-template-columns:repeat(auto-fill,minmax(260px,1fr));gap:14px}.pcard[data-v-20adbc50]{display:flex;flex-direction:column;min-height:190px}.diff[data-v-20adbc50]{margin:8px 0}.star[data-v-20adbc50]{color:#d8dbe3}.star.on[data-v-20adbc50]{color:#f59e0b}.badge.won[data-v-20adbc50]{background:#dcfce7;color:#166534}.badge.lost[data-v-20adbc50]{background:#fee2e2;color:#991b1b}.badge.not_tried[data-v-20adbc50]{background:#eef2ff;color:#4338ca}.guide[data-v-20adbc50]{background:#eef2ff;border-color:#c7d2fe;margin:12px 0 16px}