test: cover poster page tiling calculations

[strongdan]

Summary

• add focused unit coverage for A4 point conversion and orientation swapping
• cover tile-count fit tolerance, overflow, multi-sheet, and overlap behavior
• verify explicit and automatic grid orientation, count invariants, and deterministic ties
• cover A-series labels in both orientations and custom-size rounding

No production code was changed; the existing helpers satisfy the specified behavior.

Verification

• npm test -- src/lib/paper.test.ts — 14 tests passed
• npm run typecheck — passed

Closes #18

[McanKul]

Thanks again, Dan — this is a focused and thorough second contribution. The boundary, overlap, orientation, and size-label cases all look good; I also verified the targeted tests, full frontend suite, typecheck, and production build locally. Approved!