Implement stuck snake mechanics, persistent colors, and length display

Major gameplay changes:
- Snakes no longer die from collisions
- When blocked, snakes get "stuck" - head stays in place, tail shrinks by 1 per tick
- Snakes auto-unstick when obstacle clears (other snakes move/shrink away)
- Minimum snake length is 1 (head-only)
- Game runs continuously without rounds or game-over state

Color system:
- Each player gets a persistent color for their entire connection
- Colors assigned on join, rotate through available colors
- Color follows player even after disconnect/reconnect
- Works for both desktop and web clients

Display improvements:
- Show snake length instead of score
- Length accurately reflects current snake size
- Updates in real-time as snakes grow/shrink

Server fixes:
- Fixed HTTP server initialization issues
- Changed default host to 0.0.0.0 for network multiplayer
- Improved file serving with proper routing

Testing:
- Updated all collision tests for stuck mechanics
- Added tests for stuck/unstick behavior
- Added tests for color persistence
- All 12 tests passing

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

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

src/shared/models.py

@@ -32,6 +32,8 @@ class Snake:
     direction: Tuple[int, int] = (1, 0)  # Default: moving right
     alive: bool = True
     score: int = 0
+    stuck: bool = False  # True when snake is blocked and shrinking
+    color_index: int = 0  # Index in COLOR_SNAKES array for persistent color
 
     def get_head(self) -> Position:
         """Get the head position of the snake."""
@@ -45,6 +47,8 @@ class Snake:
             "direction": self.direction,
             "alive": self.alive,
             "score": self.score,
+            "stuck": self.stuck,
+            "color_index": self.color_index,
         }
 
     @classmethod
@@ -55,6 +59,8 @@ class Snake:
         snake.direction = tuple(data["direction"])
         snake.alive = data["alive"]
         snake.score = data["score"]
+        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
         return snake