refactor: Spread game code into modular files for better maintainability

This commit reorganizes the game's source code into multiple files within a `js/` directory, creating a more modular and maintainable structure. The changes include:

- Created separate files for different game components:
  - `constants.js`: Game constants and element types
  - `world.js`: World management functions
  - `terrain.js`: Terrain generation logic
  - `physics.js`: Physics simulation
  - `render.js`: Rendering functions
  - `input.js`: Input handling
  - `main.js`: Main game initialization and loop
  - Element-specific files in `js/elements/`:
    - `basic.js`: Sand, water, dirt behaviors
    - `plants.js`: Grass, seeds, flowers
    - `trees.js`: Tree growth and leaf generation
    - `fire.js`: Fire and lava behaviors

- Updated `index.html` to load modules in the correct order
- Removed the monolithic `script.js`

The modular approach improves code readability, makes future extensions easier, and separates concerns more effectively.

js/main.js

@@ -0,0 +1,76 @@
+// Main game variables and initialization
+let canvas, ctx;
+let currentTool = SAND;
+let lastFrameTime = 0;
+let fps = 0;
+let debugMode = false;
+
+// Initialize the simulation
+window.onload = function() {
+    canvas = document.getElementById('simulation-canvas');
+    ctx = canvas.getContext('2d');
+    
+    // Set canvas size to fill the screen
+    resizeCanvas();
+    window.addEventListener('resize', resizeCanvas);
+    
+    // Tool selection
+    document.getElementById('sand-btn').addEventListener('click', () => setTool(SAND));
+    document.getElementById('water-btn').addEventListener('click', () => setTool(WATER));
+    document.getElementById('dirt-btn').addEventListener('click', () => setTool(DIRT));
+    document.getElementById('stone-btn').addEventListener('click', () => setTool(STONE));
+    document.getElementById('grass-btn').addEventListener('click', () => setTool(GRASS));
+    document.getElementById('wood-btn').addEventListener('click', () => setTool(WOOD));
+    document.getElementById('seed-btn').addEventListener('click', () => setTool(SEED));
+    document.getElementById('tree-seed-btn').addEventListener('click', () => setTool(TREE_SEED));
+    document.getElementById('fire-btn').addEventListener('click', () => setTool(FIRE));
+    document.getElementById('lava-btn').addEventListener('click', () => setTool(LAVA));
+    document.getElementById('eraser-btn').addEventListener('click', () => setTool(EMPTY));
+    
+    // Navigation controls
+    document.getElementById('move-left').addEventListener('click', () => moveWorld(-CHUNK_SIZE/2, 0));
+    document.getElementById('move-right').addEventListener('click', () => moveWorld(CHUNK_SIZE/2, 0));
+    document.getElementById('move-up').addEventListener('click', () => moveWorld(0, -CHUNK_SIZE/2));
+    document.getElementById('move-down').addEventListener('click', () => moveWorld(0, CHUNK_SIZE/2));
+    document.getElementById('debug-btn').addEventListener('click', toggleDebug);
+    
+    // Drawing events
+    canvas.addEventListener('mousedown', handleMouseDown);
+    canvas.addEventListener('mousemove', handleMouseMove);
+    canvas.addEventListener('mouseup', handleMouseUp);
+    canvas.addEventListener('mouseleave', handleMouseUp);
+    
+    // Touch events for mobile
+    canvas.addEventListener('touchstart', handleTouchStart);
+    canvas.addEventListener('touchmove', handleTouchMove);
+    canvas.addEventListener('touchend', handleMouseUp);
+    
+    // Initialize the first chunk and generate terrain around it
+    getOrCreateChunk(0, 0);
+    generateChunksAroundPlayer();
+    
+    // Start the simulation loop
+    requestAnimationFrame(simulationLoop);
+};
+
+function resizeCanvas() {
+    canvas.width = window.innerWidth;
+    canvas.height = window.innerHeight - document.querySelector('.controls').offsetHeight;
+}
+
+function simulationLoop(timestamp) {
+    // Calculate FPS
+    const deltaTime = timestamp - lastFrameTime;
+    lastFrameTime = timestamp;
+    fps = Math.round(1000 / deltaTime);
+    document.getElementById('fps').textContent = `FPS: ${fps}`;
+    
+    // Update physics
+    updatePhysics();
+    
+    // Render
+    render();
+    
+    // Continue the loop
+    requestAnimationFrame(simulationLoop);
+}