feat: replace procedural fish with GLB 3D models + tank decor

- 6 tropical fish (angelfish x2, goldfish x2, guppy x2) from maya's-aquarium-3d
- 4 Quaternius cute fish (Fish1 x2, Fish2, Fish3) with swim animations
- 2 plants + 3 rocks on the gravel floor
- Wandering AI with frame-rate-independent smoothing
- Quaternius fish rotated to match local +X forward axis
- .gitignore: add .env

.gitignore

@@ -1,2 +1,3 @@
 node_modules
 dist
+.env