Add A3 and Tabloid paper sizes

[noobydp]

Summary

• Add A3 and Tabloid as supported paper-size presets for trace calibration.
• Keep backend constants, request/session schemas, frontend types, and the trace-page selector in sync.
• Preserve saved session paper size when reloading the trace page so A3/Tabloid sessions do not fall back to A4.
• Fix corrected-image downscale ratio reporting to use the actual integer resize ratio.
• Update user-facing docs and onboarding copy for the expanded paper-size list.

Validation

• git diff --check
• D:\Development\tracefinity\backend\venv\Scripts\python.exe -m pytest backend\tests\test_paper_sizes.py backend\tests\test_image_downscale.py -q
  • 8 passed
• corepack pnpm@10.29.2 --dir frontend exec tsc --noEmit
• corepack pnpm@10.29.2 --dir frontend build

Code and PR drafted with Codex

[jasonmadigan]

nice @noobydp, thank you

stale comment in image_processor.py (aspect ratio list) -- can fix in a follow-up.