fix: TypeScript build errors in Docker

- useRef(null) initial values for Strict TS 6.0
- Removed deprecated transparent option in pixi v7
- Cast Live2DModel to any for addChild type compat
- Cast coreModel for getTexture access
- Fixed types/index.ts import path to scenes
- Added vite-env.d.ts with CSS module declarations
- Added null-coalesce for clearInterval calls

frontend/src/types/index.ts

@@ -1,4 +1,4 @@
-import { SCENES, type Scene } from './scenes';
+import { SCENES, type Scene } from '../components/scenes';
 
 export interface KiraState {
   currentScene: Scene;