fix: keep media ZIP exports within import limits

[agammann]

Problem

“Export with photos & videos” could write a ZIP that “Import backup” rejects. At commit 31c6795, 1,999 distinct media files produced 2,001 ZIP entries (backup JSON, media, and README). Export reported 1,999 included files, but importing that ZIP failed with too-many.

Change

Make the ZIP writer enforce the reader’s 2,000-entry and 1 GiB entry-data limits. Exports beyond those limits now fail instead of producing a backup the app cannot restore.

Verification

• Reproduced the 1,999-file failure against 31c6795.
• pnpm exec vitest run src/lib/zip.test.js src/lib/backup-media.test.js — 13 passed.
• pnpm exec vitest run — 3,163 passed across 256 files.
• In the local guest UI, exported and reimported a ZIP with a custom exercise image; the exercise and image were restored.