chore: code quality audit and lint fixes

- Fix all 63 ESLint errors across codebase
- Consolidate MaterialWithTextureSlots type in src/types/three/three.ts
- Add CSS custom properties for design tokens
- Extract ebike constants to src/data/ebike/ebikeConfig.ts
- Add proper TypeScript types for window extensions
- Fix React hooks violations (refs during render, setState in effects)
- Remove unused exports and redundant CSS
- Add type guards for Three.js material handling
- Clean up AI slop comments and legacy CSS patterns

src/types/ebike/ebikeWindow.ts

@@ -0,0 +1,11 @@
+import type * as THREE from "three";
+import type { Vector3Tuple } from "@/types/three/three";
+
+declare global {
+  interface Window {
+    ebikeVisualGroup: React.RefObject<THREE.Group | null> | null;
+    ebikeParkedPosition: Vector3Tuple | null;
+    ebikeParkedRotation: number | null;
+    ebikeSteerFactor: number | undefined;
+  }
+}