Fix PDF center tap controls

[codedogQBY]

Summary

• Map PDF iframe click coordinates back to the reader container before deciding click zones
• Avoid sending an invalid left-edge fallback for fixed-layout pages
• Guard fixed-layout location reporting before the first spread is ready

Fixes #291

Test

• pnpm --filter app build
• git diff --check

Note: pnpm exec biome check packages/app/src/components/reader/FoliateViewer.tsx packages/foliate-js/fixed-layout.js still reports existing lint debt in these files unrelated to this change.