feat: upgrade max upload size to 5 GB and add rolling limit env var

[dobby-coder]

Summary

Implements the frontend configuration changes for the new upload limits:

• VITE_MAX_UPLOAD_SIZE: bumped from 2 GiB (2,147,483,648 bytes) to 5 GB (5,000,000,000 bytes)
• VITE_ROLLING_LIMIT: new env var set to 5 GB (5,000,000,000 bytes) — 5 GB per 2 weeks per email address
• Exported ROLLING_LIMIT from src/lib/env.ts so the frontend (and the upcoming UI warning work in Show clear warnings and errors when approaching or hitting upload limits #87) can consume it
• Switched display calculations from binary GiB (1024^3) to decimal GB (1e9) for consistency with the "GB" label shown in the UI
• Updated all hardcoded "2 GB" strings in locale files (EN and NL) to "5 GB"
• Documented VITE_ROLLING_LIMIT in README

Related issues

• Closes Upgrade max upload size to 5 GB with 5 GB per 2-week rolling limit #85
• Related: Add file size limits as primary sales pitch on the frontpage #86 (marketing frontpage — should use the same numbers)
• Related: Show clear warnings and errors when approaching or hitting upload limits #87 (UI warnings/errors — will consume ROLLING_LIMIT and the backend usage endpoint)
• Related: Enforce server-side upload limits: 5 GB rolling per email (14 days) cryptify#100 (server-side enforcement)

Notes

• The ROLLING_LIMIT is exported and available but not yet consumed in the UI — that work belongs to Show clear warnings and errors when approaching or hitting upload limits #87, which will wire it up to the /usage endpoint from cryptify#100
• Display math was changed from binary to decimal: MAX_UPLOAD_SIZE / (1024 ** 3) → MAX_UPLOAD_SIZE / 1e9. This ensures "5 GB" displays as "5.00" and not "4.66"
• Build and type check both pass cleanly

Verification

npm run check   # 0 errors, 0 warnings
npm run build   # succeeds, 5000000000 baked into output

[rubenhensen]

Superseded by #110 which combines this with #90 and #91.