feat: implement MIP-04 encrypted media upload for Marmot groups

Adds full MIP-04 (Encrypted Media) support for Marmot group chats:

Protocol layer (quartz):
- Mip04MediaEncryption: ChaCha20-Poly1305 AEAD with MLS exporter key
  derivation (HKDF-Expand), random nonces, and AAD binding
- Mip04Cipher/Mip04NostrCipher: NostrCipher adapters for the existing
  EncryptedBlobInterceptor download-and-decrypt pipeline
- Mip04IMetaTag: NIP-92 imeta tag builder/parser with MIP-04 fields
  (url, m, filename, x, n, v=mip04-v2)
- MlsGroupManager.mediaExporterSecret(): MLS-Exporter("marmot",
  "encrypted-media", 32)

Upload flow (amethyst):
- MarmotFileUploader: per-file Mip04NostrCipher → existing
  UploadOrchestrator.uploadEncrypted pipeline (compression, stripping,
  Blossom upload)
- MarmotFileSender: builds imeta tags and sends kind:9 inner events
  through the MLS group message pipeline
- MarmotGroupMessageComposer: adds SelectFromGallery leading icon and
  ChatFileUploadDialog (mirrors NIP-17 DM pattern)

Display flow:
- RenderMarmotEncryptedMedia: parses imeta v=mip04-v2 tags, derives
  Mip04Cipher, registers in EncryptionKeyCache, renders via
  ZoomableContentView (same path as NIP-17 encrypted files)
- MarmotGroupList.noteToGroupIndex: reverse index for mapping notes
  back to their group ID (needed for exporter secret lookup)
- ChatMessageCompose NoteRow: routes MIP-04 events to the new renderer

https://claude.ai/code/session_01TckzZLpJdmE6p198DHBQ5i

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/marmot/MarmotManager.kt

@@ -403,6 +403,12 @@ class MarmotManager(
      */
     fun groupEpoch(nostrGroupId: HexKey): Long? = groupManager.getGroup(nostrGroupId)?.epoch
 
+    /**
+     * Get the MIP-04 media exporter secret for a group.
+     * MLS-Exporter("marmot", "encrypted-media", 32)
+     */
+    fun mediaExporterSecret(nostrGroupId: HexKey): ByteArray = groupManager.mediaExporterSecret(nostrGroupId)
+
     /**
      * Get the MIP-01 group metadata from the MLS GroupContext extensions.
      * Returns null if the group doesn't exist or has no MarmotGroupData extension.

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/marmotGroups/MarmotGroupList.kt

@@ -36,6 +36,8 @@ class MarmotGroupList(
     var rooms = LargeCache<HexKey, MarmotGroupChatroom>()
         private set
 
+    private val noteToGroupIndex = LargeCache<HexKey, HexKey>()
+
     private val _groupListChanges = MutableSharedFlow<HexKey>(0, 20, BufferOverflow.DROP_OLDEST)
     val groupListChanges = _groupListChanges
 
@@ -56,6 +58,7 @@ class MarmotGroupList(
                 chatroom.addMessageSync(msg)
             }
         if (added) {
+            noteToGroupIndex.getOrCreate(msg.idHex) { nostrGroupId }
             if (isSelfAuthored) {
                 chatroom.ownerSentMessage = true
             }
@@ -73,6 +76,7 @@ class MarmotGroupList(
     ) {
         val chatroom = getOrCreateGroup(nostrGroupId)
         if (chatroom.restoreMessageSync(msg)) {
+            noteToGroupIndex.getOrCreate(msg.idHex) { nostrGroupId }
             if (msg.author?.pubkeyHex == ownerPubKey) {
                 chatroom.ownerSentMessage = true
             }
@@ -80,6 +84,8 @@ class MarmotGroupList(
         }
     }
 
+    fun groupIdForNote(noteId: HexKey): HexKey? = noteToGroupIndex.get(noteId)
+
     /**
      * Mark a group as "known" by the local user — used right after the user
      * creates a group, so the creator doesn't appear under "New Requests"