Fix import casing mismatches for placeholder components

Created by: jedmund

Summary

• Corrects AudioPlaceHolder → AudioPlaceholder, IFramePlaceHolder → IFramePlaceholder, and VideoPlaceHolder → VideoPlaceholder import paths and variable names in editor.svelte
• Fixes 2 TypeScript errors caused by filename casing mismatches between imports and actual files on disk

Test plan

• Verify pnpm run check no longer reports casing mismatch errors for these files
• Verify editor placeholder components still render correctly