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/constants.js

@@ -1,10 +1,11 @@
 // Globale Konstanten und Block-/Biome-Definitionen
 
 // --- Welt-Dimensionen ---
+// Hinweis: SEA_LEVEL und WORLD_SEED sind einstellbar und liegen in config.js.
+// Chunk-Größen sind strukturell (Array-/Mesh-Größen) und sollten NICHT geändert
+// werden.
 export const CHUNK_SIZE = 16;        // X und Z Ausdehnung eines Chunks
 export const CHUNK_HEIGHT = 64;      // Y Ausdehnung eines Chunks
-export const SEA_LEVEL = 26;
-export const WORLD_SEED = 1337;
 
 // --- Block-Typen ---
 export const BLOCK = {