Add file encryption for conversation attachments

[joepio]

Before: Encrypted conversations could seal text, but there was no way to encrypt a file, so a file could not be attached without the host seeing it.

After: A file can be encrypted on the device with its own random key. The key never leaves the sealed message, and a ciphertext cannot be moved into another conversation. Nothing in the app uses it yet; the next PR adds attachments to the chat.

How: seal_file and open_file in lib/src/conversation.rs use XChaCha20-Poly1305 with a random 32-byte key and a random 24-byte nonce per file, laid out as version, nonce, ciphertext and tag. The associated data holds a fixed label plus the conversation subject. The wasm crate exports conversationSealFile and conversationOpenFile, with sealFile and openFile wrappers in conversationCrypto.ts. Seven new Rust tests cover a round trip, a wrong conversation, a wrong key, a flipped byte in each part, two seals of the same bytes differing, an empty file and a 25 MiB file (19 conversation tests pass, clippy and fmt are clean for lib and the wasm32 target). Step 1 of the DM hosting plan.

Requested by Joep

[joepio]

Landed in develop through the batch #2210 (squash f39e460), tested together on Mancave run 5204.

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