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/physics.js

@@ -0,0 +1,53 @@
+// Physics simulation functions
+function updatePhysics() {
+    // Get visible chunks
+    const visibleChunks = getVisibleChunks();
+    
+    // Increment fire update counter
+    fireUpdateCounter++;
+    
+    // Process each visible chunk
+    for (const { chunkX, chunkY } of visibleChunks) {
+        const chunk = getOrCreateChunk(chunkX, chunkY);
+        
+        // Process from bottom to top, right to left for correct gravity simulation
+        for (let y = CHUNK_SIZE - 1; y >= 0; y--) {
+            // Alternate direction each row for more natural flow
+            const startX = y % 2 === 0 ? 0 : CHUNK_SIZE - 1;
+            const endX = y % 2 === 0 ? CHUNK_SIZE : -1;
+            const step = y % 2 === 0 ? 1 : -1;
+            
+            for (let x = startX; x !== endX; x += step) {
+                const index = y * CHUNK_SIZE + x;
+                const type = chunk[index];
+                
+                if (type === EMPTY || type === STONE || type === WOOD || type === LEAF) continue;
+                
+                const worldX = chunkX * CHUNK_SIZE + x;
+                const worldY = chunkY * CHUNK_SIZE + y;
+                
+                if (type === SAND) {
+                    updateSand(worldX, worldY);
+                } else if (type === WATER) {
+                    updateWater(worldX, worldY);
+                } else if (type === DIRT) {
+                    updateDirt(worldX, worldY);
+                } else if (type === GRASS) {
+                    updateGrass(worldX, worldY);
+                } else if (type === SEED) {
+                    updateSeed(worldX, worldY);
+                } else if (type === GRASS_BLADE) {
+                    updateGrassBlade(worldX, worldY);
+                } else if (type === FLOWER) {
+                    updateFlower(worldX, worldY);
+                } else if (type === TREE_SEED) {
+                    updateTreeSeed(worldX, worldY);
+                } else if (type === FIRE) {
+                    updateFire(worldX, worldY);
+                } else if (type === LAVA) {
+                    updateLava(worldX, worldY);
+                }
+            }
+        }
+    }
+}