✨ fix depth sorting for world objects by tileY

Fixes #31. All trees, rocks, seedlings and buildings now use
tileY+5 as depth instead of a fixed value, so objects further
down the screen always render in front of objects above them
regardless of spawn order. Build ghost moved to depth 1000/1001.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/systems/ResourceSystem.ts

@@ -47,10 +47,10 @@ export class ResourceSystem {
       sprite.setOrigin(0.5, 0.75)
     }
 
-    sprite.setDepth(5)
+    sprite.setDepth(node.tileY + 5)
 
     const healthBar = this.scene.add.graphics()
-    healthBar.setDepth(6)
+    healthBar.setDepth(node.tileY + 6)
     healthBar.setVisible(false)
 
     this.sprites.set(node.id, { sprite, node, healthBar })