Replace placeholder renderStep3Attach/attachStep3Attach with the real
attach flow: file-picker for reference JPEG, passphrase input with
visibility toggle, then fetch salt+params+manifest.enc, call
unlock()+manifest_decrypt() to AEAD-verify credentials before
advancing to Step 4. Wrong passphrase/image shows a clear error;
partial handles are locked on failure to avoid key-material leaks.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>