feat: support multiple file encryption with ZIP archive mode

[valerius21]

Summary

Adds multi-file support to Seal3D, closing #8.

• Users can now select multiple files via drag-and-drop or file picker
• When multiple files are selected in encrypt mode, a toggle appears to choose between:
  • ZIP Archive (default): bundles all files into a ZIP, encrypts it, downloads as seal3d-archive.zip.encrypted
  • Individual Files: encrypts each file separately, downloads sequentially as file.name.encrypted
• Single-file encryption/decryption behavior is unchanged (backward compatible)
• Decryption of a .zip.encrypted yields the .zip — user extracts with OS tools

Changes

File	What
package.json	Added fflate@0.8.2 (ZIP library, ~8KB gzipped, zero deps)
lib/zip.ts	New — createZipFromFiles, extractZip, verifyZip, listZipEntries
lib/zip.test.ts	New — 18 tests covering all ZIP utility functions
components/file-drop-zone.tsx	Updated — accepts File[], multi-select, file list with remove buttons
components/archive-toggle.tsx	New — toggle between archive and individual encryption modes
app/page.tsx	Updated — multi-file state, 3 encrypt paths, multi-file decrypt
lib/crypto.test.ts	Updated — 7 new integration tests for multi-file encrypt/decrypt

Test Coverage

• 47 tests passing (1 skipped: opt-in large-file test)
• zip.ts: 100% across all metrics
• crypto.ts: 99% lines
• Overall: 96.4% statements, 99.3% lines, 100% functions

Closes #8

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	seal3d	fc0b209	Commit Preview URL Branch Preview URL	Feb 20 2026, 07:12 PM

[valerius21]

needs more testing