Add input buffering, auto-start, and gameplay improvements

Input buffer system:
- Added 3-slot direction input buffer to handle rapid key presses
- Buffer ignores duplicate inputs (same key pressed multiple times)
- Opposite direction replaces last buffered input (e.g., LEFT→RIGHT replaces LEFT)
- Buffer overflow replaces last slot when full
- Multi-segment snakes skip invalid 180° turns when consuming buffer
- Head-only snakes (length=1) can turn 180° for flexibility

Gameplay improvements:
- Desktop client auto-starts game on connect (no SPACE needed)
- Field populates with 3 apples when no players connected
- HTTP server now binds to 0.0.0.0 for network access (matches game server)

Testing:
- Added 7 new tests for input buffer functionality
- Added test for zero-player apple spawning
- All 19 tests passing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/shared/models.py

@@ -35,6 +35,7 @@ class Snake:
     stuck: bool = False  # True when snake is blocked and shrinking
     color_index: int = 0  # Index in COLOR_SNAKES array for persistent color
     player_name: str = ""  # Human-readable player name
+    input_buffer: List[Tuple[int, int]] = field(default_factory=list)  # Buffer for pending direction changes (max 3)
 
     def get_head(self) -> Position:
         """Get the head position of the snake."""
@@ -51,6 +52,7 @@ class Snake:
             "stuck": self.stuck,
             "color_index": self.color_index,
             "player_name": self.player_name,
+            "input_buffer": self.input_buffer,
         }
 
     @classmethod
@@ -64,6 +66,7 @@ class Snake:
         snake.stuck = data.get("stuck", False)  # Default to False for backward compatibility
         snake.color_index = data.get("color_index", 0)  # Default to 0 for backward compatibility
         snake.player_name = data.get("player_name", "")  # Default to empty string for backward compatibility
+        snake.input_buffer = [tuple(d) for d in data.get("input_buffer", [])]  # Default to empty list for backward compatibility
         return snake