refactor: route every MLS group restore through ChatRoom.toMlsGroup()

Four places restored an MlsGroup from a chat room's persisted state, each
spelling out the same MlsGroup.restore(MlsGroupState.decodeTls(hex))
chain by hand. Three of them could not have called the shared helper
even if they wanted to: until ea8b2b2, ChatRoom.toMlsGroup() demanded a
pastGenerations count so it could replay the sender ratchet, which is a
question none of these callers had any business answering. Dropping that
parameter left the helper callable everywhere, so call it everywhere.

database/repository/DatabaseChatRepository.kt
* sendChatMessage() restores via toMlsGroup(). It only wants to know
  whether the room is MLS-backed or gift-wrapped, which is exactly what
  a null return says.

ui/view/model/AddArtifactViewModel.kt
* Same collapse: the ?.let { restore(...) }?.let { mlsGroup -> ... }
  double-let becomes toMlsGroup()?.let { mlsGroup -> ... }.

database/dao/NostrDao.kt
* The fourth copy, and the one easiest to miss: it decoded the hex into
  a mlsGroupStateByteArray temp and then restored under an explicit null
  check on the bytes, rather than the ?.let the others used. Same
  operation wearing different clothes. Now toMlsGroup() with a null
  check on the group itself, which is what the branch actually meant.

None of these three encrypt, so none of them ever needed the generation
replay the old signature forced on them -- NostrDao is the inbound path,
AddArtifactViewModel only gates on the group existing, and
sendChatMessage writes a rumor for the outbound pipeline to encrypt
later. The only caller that does encrypt, encryptAndSendMarmotInnerEvent,
persists mlsGroup.saveState() immediately afterwards, so the ratchet
position now round-trips through storage on its own.

Unused imports go with them: MlsGroup and MlsGroupState from the first
two files, MlsGroupState from NostrDao (which still needs MlsGroup for
processWelcome).

MlsGroup.restore now appears exactly once in the codebase, inside
toMlsGroup() itself.

Verified:
  ./gradlew :composeApp:compileCommonMainKotlinMetadata
  ./gradlew :composeApp:compileDebugKotlinAndroid --rerun-tasks

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

composeApp/src/commonMain/kotlin/press/mantra/compose/database/dao/NostrDao.kt

@@ -36,7 +36,6 @@ import com.vitorpamplona.quartz.marmot.mip00KeyPackages.KeyPackageRelayListEvent
 import com.vitorpamplona.quartz.marmot.mip02Welcome.WelcomeEvent
 import com.vitorpamplona.quartz.marmot.mls.codec.TlsReader
 import com.vitorpamplona.quartz.marmot.mls.group.MlsGroup
-import com.vitorpamplona.quartz.marmot.mls.group.MlsGroupState
 import com.vitorpamplona.quartz.marmot.mls.messages.KeyPackageBundle
 import com.vitorpamplona.quartz.marmot.mls.messages.MlsKeyPackage
 import com.vitorpamplona.quartz.marmot.mls.tree.Credential
@@ -382,15 +381,9 @@ abstract class NostrDao(
                 val localChatRoom = database.chatRoomDao().findChatRoomById(chatRoomId)
 
                 if (localChatRoom != null) {
-                    val mlsGroupStateByteArray = localChatRoom.chatRoom.mlsGroupState?.hexToByteArray()
-
-                    if (mlsGroupStateByteArray != null) {
-                        val mlsGroup = MlsGroup.restore(
-                            MlsGroupState.decodeTls(
-                                mlsGroupStateByteArray
-                            )
-                        )
+                    val mlsGroup = localChatRoom.chatRoom.toMlsGroup()
 
+                    if (mlsGroup != null) {
                         val memberPubkeys = mlsGroup.members().mapNotNull { (leafIndex, leafNode) ->
 
                             val pubkey = when (val cred = leafNode.credential) {

composeApp/src/commonMain/kotlin/press/mantra/compose/database/repository/DatabaseChatRepository.kt

@@ -12,8 +12,6 @@ import press.mantra.compose.database.model.intermdiate.LocalChatMessage
 import press.mantra.compose.database.model.intermdiate.LocalChatRoom
 import co.touchlab.kermit.Logger
 import com.vitorpamplona.quartz.marmot.mip02Welcome.WelcomeEvent
-import com.vitorpamplona.quartz.marmot.mls.group.MlsGroup
-import com.vitorpamplona.quartz.marmot.mls.group.MlsGroupState
 import com.vitorpamplona.quartz.nip01Core.core.HexKey
 import com.vitorpamplona.quartz.nip01Core.core.Kind
 import com.vitorpamplona.quartz.nip01Core.crypto.EventHasher
@@ -153,13 +151,7 @@ class DatabaseChatRepository(
         messageType: Kind
     ) {
         logger.i("sendChatMessage($text): $localChatRoom")
-        val mlsGroup = localChatRoom.chatRoom.mlsGroupState?.let { mlsGroupState ->
-            MlsGroup.restore(
-                MlsGroupState.decodeTls(
-                    mlsGroupState.hexToByteArray()
-                )
-            )
-        }
+        val mlsGroup = localChatRoom.chatRoom.toMlsGroup()
 
         if (mlsGroup != null) {
             // Create ChatRumor...

composeApp/src/commonMain/kotlin/press/mantra/compose/ui/view/model/AddArtifactViewModel.kt

@@ -14,8 +14,6 @@ import androidx.lifecycle.viewmodel.viewModelFactory
 import press.mantra.compose.repository.ChatRepository
 import press.mantra.compose.repository.NostrRepository
 import co.touchlab.kermit.Logger
-import com.vitorpamplona.quartz.marmot.mls.group.MlsGroup
-import com.vitorpamplona.quartz.marmot.mls.group.MlsGroupState
 import com.vitorpamplona.quartz.nip01Core.core.HexKey
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.IO
@@ -73,13 +71,7 @@ class AddArtifactViewModel(
         onSuccess: (artifactId: String) -> Unit,
         onFailure: () -> Unit
     ) {
-        localChatRoom.chatRoom.mlsGroupState?.let { mlsGroupState ->
-            MlsGroup.restore(
-                MlsGroupState.decodeTls(
-                    mlsGroupState.hexToByteArray()
-                )
-            )
-        }?.let { mlsGroup ->
+        localChatRoom.chatRoom.toMlsGroup()?.let { mlsGroup ->
             val name = nameField.text.toString()
             val url = urlField.text.toString()
             val versionLabel = versionLabelField.text.toString()