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>

index.html

@@ -9,6 +9,7 @@
   {
     "imports": {
       "three": "https://cdn.jsdelivr.net/npm/three@0.165.0/build/three.module.js",
+      "three/addons/": "https://cdn.jsdelivr.net/npm/three@0.165.0/examples/jsm/",
       "simplex-noise": "https://cdn.jsdelivr.net/npm/simplex-noise@4.0.1/dist/esm/simplex-noise.js"
     }
   }
@@ -22,7 +23,7 @@
       <p>Klicke, um zu starten.</p>
       <div class="keys">
         <div><b>WASD</b> Laufen &nbsp; <b>Leertaste</b> Springen</div>
-        <div><b>Maus</b> Umsehen &nbsp; <b>Shift</b> Sneak / Runter (Fly)</div>
+        <div><b>Maus</b> Umsehen &nbsp; <b>Shift</b> Sprint / Runter (Fly)</div>
         <div><b>Linke Maustaste</b> Abbauen &nbsp; <b>Rechte Maustaste</b> Setzen</div>
         <div><b>F</b> Flug-Modus &nbsp; <b>1–6</b> Block wählen &nbsp; <b>Esc</b> Pause</div>
       </div>