feat(media): encrypted file sharing in desktop DMs (NIP-17 Phase 6)

Add full send + receive encrypted media support in desktop DM chat:
- Paperclip attach button and drag-and-drop in ChatPane (NIP-17 mode only)
- AES-GCM encryption before upload to Blossom server
- ChatMessageEncryptedFileHeaderEvent (kind 15) wrapped in GiftWrap
- sendNip17EncryptedFile() added to IAccount interface and implementations
- DesktopUploadOrchestrator.uploadEncrypted() with proper encrypted hash
- DesktopBlossomClient ByteArray upload overload for encrypted blobs
- LRU cache in EncryptedMediaService to avoid re-downloading
- Error handling: retry on failure, disable send during upload

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/IAccount.kt

@@ -24,6 +24,7 @@ import com.vitorpamplona.amethyst.commons.model.privateChats.ChatroomList
 import com.vitorpamplona.quartz.nip01Core.signers.EventTemplate
 import com.vitorpamplona.quartz.nip01Core.signers.NostrSigner
 import com.vitorpamplona.quartz.nip04Dm.messages.PrivateDmEvent
+import com.vitorpamplona.quartz.nip17Dm.files.ChatMessageEncryptedFileHeaderEvent
 import com.vitorpamplona.quartz.nip17Dm.messages.ChatMessageEvent
 import com.vitorpamplona.quartz.nip47WalletConnect.LnZapPaymentRequestEvent
 import com.vitorpamplona.quartz.nip47WalletConnect.LnZapPaymentResponseEvent
@@ -108,6 +109,9 @@ interface IAccount {
     /** Send a NIP-17 gift-wrapped direct message */
     suspend fun sendNip17PrivateMessage(template: EventTemplate<ChatMessageEvent>)
 
+    /** Send a NIP-17 gift-wrapped encrypted file header */
+    suspend fun sendNip17EncryptedFile(template: EventTemplate<ChatMessageEncryptedFileHeaderEvent>)
+
     /** Broadcast pre-created gift wraps (e.g. reactions within group DMs) */
     suspend fun sendGiftWraps(wraps: List<GiftWrapEvent>)
 }