fix: support backup and restore of large projects - #491

Summary

Backing up or restoring a project with enough inlined assets failed with Invalid string length. The export was built as a single JSON.stringify string (and rebuilt via JSON.parse on restore), which exceeds V8's ~512MB max string length once asset files are embedded as base64. This fixes both directions so large projects can be backed up and restored, including password-encrypted backups.

Changes

• Serialize exports as concatenated Buffer chunks (serializeExportToBuffer), stringifying the manifest, each table row, and each asset file individually — a Buffer isn't bound by the string cap. Output is byte-identical to JSON.stringify.
• Parse imports by byte-scanning the decompressed buffer (parseExportFromBuffer), JSON.parse-ing the manifest, each row, and each file from small slices, so no large string is ever built. Safe because JSON structural characters are ASCII and don't collide with UTF-8 content.
• Encryption/decryption is unchanged and buffer-only, so encrypted backups benefit from both fixes.
• Existing .ycode files remain compatible (standard JSON, key-order/whitespace agnostic); invalid files still surface Invalid backup file.

Reported in #490.

Test plan

• Back up a small project, restore it, confirm identical result
• Back up a project with many/large assets (previously errored), confirm the .ycode downloads without Invalid string length
• Restore that large backup and verify pages, collections, and assets are intact
• Repeat backup + restore with a password and confirm decryption works
• Restore a backup created by a previous version (backward compatibility)
• Attempt to restore a corrupted/invalid file and confirm the Invalid backup file error