feat(chats): per-message encryption badge — lock for NIP-17, lock-open for NIP-04

Show a small lock icon next to the timestamp on each DM message:
- NIP-17 (ChatMessageEvent, ChatMessageEncryptedFileHeaderEvent): filled
  lock in primary color — relay can't see sender/recipient
- NIP-04 (PrivateDmEvent): open lock in muted gray — legacy encryption,
  metadata visible to relays

Matches Android's IncognitoBadge pattern. Both NIP-04 and NIP-17 messages
in the same 1-on-1 conversation produce identical ChatroomKeys, so they
merge into one conversation — the badge is the only way to tell them apart.

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

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/ui/chats/ChatPane.kt

@@ -523,6 +523,29 @@ private fun MessageWithReactions(
                         verticalAlignment = Alignment.CenterVertically,
                         horizontalArrangement = Arrangement.spacedBy(4.dp),
                     ) {
+                        // Encryption badge
+                        when (event) {
+                            is PrivateDmEvent -> {
+                                Icon(
+                                    Icons.Default.LockOpen,
+                                    contentDescription = "NIP-04 (legacy)",
+                                    modifier = Modifier.size(12.dp),
+                                    tint = MaterialTheme.colorScheme.onSurfaceVariant.copy(alpha = 0.4f),
+                                )
+                            }
+
+                            is com.vitorpamplona.quartz.nip17Dm.messages.ChatMessageEvent,
+                            is ChatMessageEncryptedFileHeaderEvent,
+                            -> {
+                                Icon(
+                                    Icons.Default.Lock,
+                                    contentDescription = "NIP-17 (encrypted)",
+                                    modifier = Modifier.size(12.dp),
+                                    tint = MaterialTheme.colorScheme.primary.copy(alpha = 0.6f),
+                                )
+                            }
+                        }
+
                         // Timestamp
                         note.createdAt()?.let { timestamp ->
                             Text(