feat(game): official first-move tile draw + admin step-by-step replay

Decide who moves first by the official rule: each seated player draws one
tile, the one closest to "A" leads (a blank beats every letter), ties
re-drawing until a single leader remains. Each draw uses honest per-draw
crypto/rand entropy (not the deterministic bag seed), so the recorded draw —
not a seed — is the only account of the outcome. The leader takes seat 0, so
the engine and journal replay are unchanged.

The draw is recorded with the game (game_setup_draws, migration 00013) for
future tournaments, designed as a discrete "player N draws a tile" step.
Friend/AI games draw at creation. Auto-match draws when the game opens,
against a synthetic uuid.Nil opponent whose draw rows (NULL account_id) are
back-filled to the real opponent on join — so the opener's seat is fixed up
front and the existing open-game pre-move is preserved with no reseating.

Admin /_gm/games/:id gains the recorded draw list and a simple step-by-step
board replay (game.ReplayTimeline + a vanilla-JS stepper): a board with
A-O/1-15 headers and highlighted premium squares, placed letters with their
tile value as a subscript, rack panels around the board (seat 0 top, 1
bottom, 2 left, 3 right) with the current player highlighted, and a per-move
log with the tiles drawn and the bag remainder.

Docs: ARCHITECTURE §6/§9, FUNCTIONAL (+_ru), PRERELEASE (FM row), design spec.

backend/internal/inttest/dictionary_update_test.go

@@ -134,11 +134,13 @@ func TestDictionaryUpdateFlow(t *testing.T) {
 // newGameServiceOn builds a game service over the shared pool but a caller-supplied
 // dictionary directory, version and registry, for the isolated dictionary tests.
 func newGameServiceOn(dir, version string, reg *engine.Registry) *game.Service {
-	return game.NewService(
+	svc := game.NewService(
 		game.NewStore(testDB), account.NewStore(testDB), reg,
 		game.Config{DictDir: dir, DictVersion: version, TimeoutSweepInterval: time.Minute, CacheTTL: time.Hour},
 		zap.NewNop(),
 	)
+	svc.SetFirstMoveEntropy(seatZeroFirstMove)
+	return svc
 }
 
 // seedDawgs copies the committed DAWG of every variant from src into dst (flat).