feat: encrypt DM media with AES-256-GCM (Kind 15)

[barrydeen]

Summary

• Files shared in DMs are now encrypted client-side with AES-256-GCM before uploading to Blossom
• Decryption key is distributed inside the NIP-17 gift wrap as a Kind 15 rumor, so only conversation participants can view the media
• Single encrypted blob per file — all recipients share the same decryption key via their respective gift wraps

Changes

• EncryptedMedia.kt (new) — AES-256-GCM encrypt/decrypt, Kind 15 tag building and parsing
• Nip17.kt — Accept kind 15 in unwrap, auto-add p-tags, isFileMessage() helper
• DmConversation.kt — encryptedFileMetadata field on DmMessage
• BlossomRepository.kt — uploadEncryptedMedia() encrypts then uploads via /upload only (skips /media which would corrupt ciphertext)
• EventRouter / DmListViewModel / DmConversationViewModel — Parse Kind 15 tags when processing received gift wraps
• DmConversationViewModel.kt — uploadMedia() now encrypts and sends Kind 15 via sendFileMessage() (local + remote signer)
• DmBubble.kt — EncryptedMediaContent composable: download, decrypt, display with in-memory LRU cache

Test plan

• Send encrypted image from Wisp, verify it appears correctly on the receiver's end
• Receive encrypted image from another Kind 15-capable client, verify Wisp decrypts and displays it
• Verify sender can see their own sent media (self-copy via gift wrap)
• Verify group DMs: all participants can view the encrypted media
• Verify non-DM media uploads (compose screen) still work unencrypted
• Verify /upload endpoint is used (not /media) for encrypted blobs