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/vite-env.d.ts

@@ -0,0 +1,11 @@
+/// <reference types="vite/client" />
+
+declare module '*.css' {
+  const content: Record<string, string>;
+  export default content;
+}
+
+declare module '*.module.css' {
+  const classes: { readonly [key: string]: string };
+  export default classes;
+}