Implement Texas Hold'em play: dealing, betting, side pots, showdown

- Hand engine (server/hand.js): crypto-shuffled deck kept server-side,
  blinds (heads-up button = SB), min-raise rules incl. incomplete all-in,
  uncalled bet refunds, side pots, split pots, all-in runout
- Hand evaluator (server/cards.js) and node:test suite (npm test)
- Table flow: hands auto-continue after host START, 30s action clock
  (timeout = check/fold + sit out), sit out / sit in, rebuy, mid-hand leave
  folds and keeps the seat reserved until the hand ends, pause freezes all clocks
- Only each player's own hole cards are sent until a showdown/runout reveal;
  all actions validated server-side
- Play UI: board, hole cards, bets, action tags, turn timer, raise panel with
  presets, showdown results with winning cards highlighted, compact 7+ layout
- Remove the host crown and the temporary "next phase" control

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/assets/common.js

@@ -92,7 +92,7 @@
     FLOP: 'FLOP',
     TURN: 'TURN',
     RIVER: 'RIVER',
-    SHOWDOWN: 'SHOWDOWN',
+    FINISHING: 'FINISHING',
   };
 
   window.App = { Store, esc, chips, toast, api, connectSocket, safeNext, PHASE_LABELS };