feat: add configurable speed and expanded grid presets for snake

- Add per-game speed setting with presets (Slow/Normal/Fast/Insane)
- Add speed selector UI in snake lobby
- Expand grid presets with Tiny (15x15) and XL (50x30)
- Auto-calculate cell size based on grid dimensions
- Preserve speed setting in rematch games

snake/loop.go

@@ -5,13 +5,11 @@ import (
 )
 
 const (
-	targetFPS                      = 60
-	tickInterval                   = time.Second / targetFPS
-	snakeSpeed                     = 7 // cells per second
-	moveInterval                   = time.Second / snakeSpeed
-	countdownSecondsMultiplayer    = 10
-	countdownSecondsSinglePlayer   = 3
-	inactivityLimit                = 60 * time.Second
+	targetFPS                    = 60
+	tickInterval                 = time.Second / targetFPS
+	countdownSecondsMultiplayer  = 10
+	countdownSecondsSinglePlayer = 3
+	inactivityLimit              = 60 * time.Second
 )
 
 func (si *SnakeGameInstance) startOrResetCountdownLocked() {
@@ -114,18 +112,13 @@ func (si *SnakeGameInstance) gamePhase() {
 				return
 			}
 
-			// Apply pending directions every tick for responsive input
-			inputReceived := false
+			// Track input activity for inactivity timeout
 			for i := 0; i < 8; i++ {
-				if si.pendingDir[i] != nil && i < len(si.game.State.Snakes) && si.game.State.Snakes[i] != nil {
-					si.game.State.Snakes[i].Dir = *si.pendingDir[i]
-					si.pendingDir[i] = nil
-					inputReceived = true
+				if len(si.pendingDirQueue[i]) > 0 {
+					lastInput = time.Now()
+					break
 				}
 			}
-			if inputReceived {
-				lastInput = time.Now()
-			}
 
 			// Inactivity timeout
 			if time.Since(lastInput) > inactivityLimit {
@@ -138,7 +131,14 @@ func (si *SnakeGameInstance) gamePhase() {
 				return
 			}
 
-			// Only advance game state at snakeSpeed
+			// Compute move interval from per-game speed
+			speed := si.game.Speed
+			if speed <= 0 {
+				speed = DefaultSpeed
+			}
+			moveInterval := time.Second / time.Duration(speed)
+
+			// Only advance game state at game speed
 			moveAccum += tickInterval
 			if moveAccum < moveInterval {
 				si.gameMu.Unlock()
@@ -146,6 +146,14 @@ func (si *SnakeGameInstance) gamePhase() {
 			}
 			moveAccum -= moveInterval
 
+			// Pop one direction from queue per movement frame
+			for i := 0; i < 8; i++ {
+				if len(si.pendingDirQueue[i]) > 0 && i < len(si.game.State.Snakes) && si.game.State.Snakes[i] != nil {
+					si.game.State.Snakes[i].Dir = si.pendingDirQueue[i][0]
+					si.pendingDirQueue[i] = si.pendingDirQueue[i][1:]
+				}
+			}
+
 			state := si.game.State
 
 			// Advance snakes