P21: player seats + two-way live play + offline handoff

Players claim a seat (GM approves) and drive their own character live —
HP/slots/resources/conditions/dice broadcast to the table — via new seat
protocol messages and per-seat ownership enforced in RoomHub. GM persists
player edits (charactersRepo.update → liveQuery → rebroadcast). Offline dev:
"Share with player" encodes a claim link (src/lib/sync/playerLink.ts) → /player
imports the character locally for full-sheet editing, bundled back as an
offlineSnapshot on seat claim for GM review.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/index.ts

@@ -45,6 +45,10 @@ export function buildServer() {
           case 'state': hub.state(sender, m.gmSecret, m.snapshot); break;
           case 'image': hub.image(sender, m.gmSecret, m.id, m.dataUrl); break;
           case 'requestImage': hub.requestImage(sender, m.id); break;
+          case 'claimSeat': hub.claimSeat(sender, m.characterId, m.offlineSnapshot); break;
+          case 'seatGrant': hub.seatGrant(sender, m.gmSecret, m.targetPlayerId, m.character); break;
+          case 'playerPatch': hub.playerPatch(sender, m.characterId, m.diff); break;
+          case 'playerRoll': hub.playerRoll(sender, m.characterId, m.label, m.expression, m.total, m.breakdown); break;
         }
       });
       socket.on('close', () => { clearInterval(refill); hub.disconnect(sender); });