feat: shadows, baked vertex AO, post-processing + central config.js

Visuals:
- PCFSoftShadowMap (2048²), directional light follows player with a tight
  shadow frustum (SHADOW_AREA) for high texel density; opak blocks cast+receive,
  water receives.
- Baked vertex ambient occlusion ("smooth lighting"): per-vertex solid-neighbor
  lookup (0fps algorithm) written into a color attribute, with flip-triangulation
  to avoid brightness seams across quad diagonals.
- EffectComposer: RenderPass -> UnrealBloomPass (subtle) -> OutputPass with
  ACES filmic tone-mapping + sRGB. HalfFloat render target (anti-banding) + MSAA.

Controls:
- Sprint moved from Ctrl to Shift (Ctrl is problematic in browsers). Shift in
  fly mode still means "descend" (no conflict, sprint only applies when walking).

config.js:
- New central js/config.js for all tweakable values (world, performance,
  renderer/tone-mapping, sky & fog, camera, lighting, shadows, bloom, AO,
  player physics, mouse sensitivity, reach) with SHADOWS_ENABLED / BLOOM_ENABLED
  feature toggles. Structural engine constants stay in constants.js.
- SEA_LEVEL / WORLD_SEED moved constants.js -> config.js (consumers rewired).

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

js/world.js

@@ -120,6 +120,8 @@ export class World {
       const m = new THREE.Mesh(opaque, this.materials.opaque);
       m.position.set(ox, 0, oz);
       m.frustumCulled = true;
+      m.castShadow = true;
+      m.receiveShadow = true;
       this.scene.add(m);
       meshes.opaque = m;
     }
@@ -127,6 +129,7 @@ export class World {
       const m = new THREE.Mesh(transparent, this.materials.water);
       m.position.set(ox, 0, oz);
       m.renderOrder = 1;
+      m.receiveShadow = true; // Wasseroberflaeche kriegt Schatten; casten unnoetig
       this.scene.add(m);
       meshes.water = m;
     }