Configure ESLint with Prettier and TypeScript support.

- Baseline existing errors to warnings.
- Fix Image to PDF tool for HEIC and WebP inputs.
- Apply linting auto-fixes.

src/js/logic/pdf-layers-page.ts

@@ -19,7 +19,7 @@ interface LayerData {
 
 let currentFile: File | null = null;
 let currentDoc: any = null;
-let layersMap = new Map<number, LayerData>();
+const layersMap = new Map<number, LayerData>();
 let nextDisplayOrder = 0;
 
 document.addEventListener('DOMContentLoaded', () => {