File encryption/decryption wired into a real file-attach UI

[codebestia]

lib/fileEncryption.ts has correct, standalone AES-256-GCM encrypt/decrypt/upload/download logic, but nothing calls it — there is no file-attach input anywhere in the live conversation UI. This depends on the session/envelope fixes, since the file's symmetric key needs to travel inside a working per-device envelope.

Acceptance criteria:

• app/app/conversations/[id]/page.tsx gets a file-attach control that calls encryptFile/requestPresignedUpload/uploadCiphertextToS3/sendEncryptedFile
• The file's decryption key travels inside the per-device envelope ciphertext, not as a separate plaintext field
• Receiving a file message calls fetchPresignedDownload/downloadAndDecryptFile to retrieve and decrypt it client-side
• Coordinated with the backend's file-message envelope migration — client and server land together
• A test exercises the encrypt→upload→download→decrypt round trip against the now-real upload endpoints

[samjay8]

@samjay8 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello Boss, I will like to work on this issue, please kindly assign to me.

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[Ssoleed]

@Ssoleed has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

hi maintainer. i would like to work on this issue please.

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[AbelOsaretin]

@AbelOsaretin has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I would love to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @AbelOsaretin to this issue.

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Congratulations, @AbelOsaretin! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

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