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>

package.json

@@ -6,7 +6,8 @@
   "main": "server/index.js",
   "scripts": {
     "start": "node server/index.js",
-    "dev": "node --watch server/index.js"
+    "dev": "node --watch server/index.js",
+    "test": "node --test"
   },
   "engines": {
     "node": ">=18"