Fix short article column layout

[taherdhanera]

/claim #32

Fixes #32.

Summary

• Change Scroll column layout to fill columns vertically before balancing into extra columns.
• Give short article column containers a viewport-height minimum so one-page articles no longer spread sparse content across too many columns.

Verification

• node -e "const fs=require('fs'); const css=fs.readFileSync('external/.scroll.css','utf8'); console.log(css.includes('column-fill: auto;') && css.includes('min-height: 100vh;') ? 'css check passed' : 'css check failed')"

Notes

• npm test and npm run build currently fail on this checkout before this CSS change because the installed scrollsdk exposes scrollProgram without the load() method expected by scroll.js.