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/constants.py

@@ -1,7 +1,7 @@
 """Game constants shared between client and server."""
 
 # Network settings
-DEFAULT_HOST = "localhost"
+DEFAULT_HOST = "0.0.0.0"  # Listen on all interfaces for multiplayer
 DEFAULT_PORT = 8888
 DEFAULT_WS_PORT = 8889
 DEFAULT_HTTP_PORT = 8000