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/worldgen.js

@@ -1,9 +1,8 @@
 // Weltgenerierung: Terrain, Biome, Hoehlen, Wasser, Baeume.
 // Arbeitet direkt auf Chunk-Block-Arrays. Baeume koennen chunk-uebergreifend
 // in Nachbarn schreiben (world.setGen / world.getGen).
-import {
-  CHUNK_SIZE, CHUNK_HEIGHT, SEA_LEVEL, BLOCK, BIOME,
-} from './constants.js';
+import { CHUNK_SIZE, CHUNK_HEIGHT, BLOCK, BIOME } from './constants.js';
+import { SEA_LEVEL } from './config.js';
 import {
   noise2DTerrain, noise2DDetail, noise2DTemp, noise2DHumid, noise3DCave, fbm2,
 } from './noise.js';