Export print-ready sRGB A4 PDF

[ankitsejwal]

Implements #8 — one-click print-ready PDF of the polaroids.

• lib/pdf.ts builds an A4, multipage PDF with pdf-lib, using the shared lib/layout.ts so it matches the on-screen preview (A4 preview canvas (Konva) #6).
• Images are centre-cropped and rasterized to sRGB JPEG via canvas — handles any format the browser can decode, and sRGB is exactly what photo labs / home inkjets (incl. the Canon G1010) want. No CMYK (deliberate — see README/research).
• 300 DPI image cells, white frames, dashed margin guide, centered captions; "Export PDF" button on the preview downloads polaroids.pdf.

Verification (headless)

Prototyped the identical layout + pdf-lib drawing and rendered the PDF to PNG — A4 size confirmed (595.3 × 841.9 pt), 3-per-row grid, real photos embedded, captions + margin guide correct. lib/pdf.ts reuses the same lib/layout.ts math. pnpm lint/typecheck/build green.

Follow-up

Captions render in Helvetica (vector, crisp); matching the handwriting font in the PDF needs font embedding — tracked in #23.

Closes #8