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/types.go

@@ -97,8 +97,24 @@ type SnakeGame struct {
 	RematchGameID *string
 	Mode          GameMode // ModeMultiplayer or ModeSinglePlayer
 	Score         int      // tracks food eaten in single player
+	Speed         int      // cells per second
 }
 
+// Speed presets
+type SpeedPreset struct {
+	Name  string
+	Speed int
+}
+
+var SpeedPresets = []SpeedPreset{
+	{Name: "Slow", Speed: 5},
+	{Name: "Normal", Speed: 7},
+	{Name: "Fast", Speed: 10},
+	{Name: "Insane", Speed: 15},
+}
+
+const DefaultSpeed = 7
+
 func (sg *SnakeGame) IsFinished() bool {
 	return sg.Status == StatusFinished
 }
@@ -121,9 +137,11 @@ type GridPreset struct {
 }
 
 var GridPresets = []GridPreset{
+	{Name: "Tiny", Width: 15, Height: 15},
 	{Name: "Small", Width: 20, Height: 20},
 	{Name: "Medium", Width: 30, Height: 20},
 	{Name: "Large", Width: 40, Height: 20},
+	{Name: "XL", Width: 50, Height: 30},
 }
 
 // snapshot returns a shallow copy of the game safe for reading outside the lock.