Fix missing photos and page stretching in PDF exports

[Dronquavious]

Summary

• Include inserted photos and typed text on their correct PDF pages, preserving image crop, rotation, and stacking order.
• Use uniform scaling and explicit Letter page dimensions to prevent stretched exports.
• Report an export error if a saved photo cannot be read.
• Add focused PDF export tests. No Mobile Ink or MathNotes code changed.

Verification

• Confirmed the reported bugs are fixed in the app.
• npm test: 56 passed
• npm run typecheck: passed
• iOS and Android Expo bundles: passed
• Rendered a three-page PDF and checked its page size and image shape.

Remaining limitations

Imported PDF pages are still rasterized onto Letter pages. The existing low-resolution preview path for notes of 150 or more pages is unchanged.