fix(marmot): persist group metadata + decrypted messages across restarts

Two related Marmot/MLS group bugs were causing data loss across app
restarts:

1. **Group name disappears, edit screen shows "Group Metadata Not Found".**
   `CreateGroupScreen` was setting `chatroom.displayName.value` directly
   in memory and never writing the name into the MLS GroupContext
   extensions. After restart `MarmotManager.syncMetadataTo` had nothing
   to read from, and `EditGroupInfoScreen` blew up because
   `AccountViewModel.updateMarmotGroupMetadata` required existing
   metadata to copy from.

   Now `CreateGroupScreen` issues a real GCE commit through
   `updateMarmotGroupMetadata` so the name is persisted in the MLS
   extensions on disk. `AccountViewModel.updateMarmotGroupMetadata`
   tolerates missing prior metadata by constructing a fresh
   `MarmotGroupData` (creator as admin), so older groups can also be
   recovered by editing them. `Account.updateMarmotGroupMetadata` now
   calls `syncMetadataTo` after the local commit so the chatroom UI
   reflects the new name without waiting for the relay round-trip.

2. **Messages disappear after restart.** `MarmotGroupChatroom.messages`
   was purely in-memory. Marmot/MLS application messages cannot be
   re-decrypted once the ratchet has advanced, so relay redelivery is
   not enough to restore history — the plaintext must be captured at
   first decryption and persisted.

   This change introduces `MarmotMessageStore` (quartz interface) and
   `AndroidMarmotMessageStore` (encrypted, file-based, sharing the same
   KeyStoreEncryption used for MLS state). `MarmotManager` now exposes
   `persistDecryptedMessage` / `loadStoredMessages` and clears the log
   on `leaveGroup`. `GroupEventHandler` persists each new application
   message after it has been added to the chatroom. On startup,
   `Account.init` loads any stored messages for restored groups and
   re-hydrates the chatroom via a new `restoreMessageSync` helper that
   does not bump the unread counter.

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/marmot/mls/group/MarmotMessageStore.kt

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.marmot.mls.group
+
+/**
+ * Encrypted local storage for decrypted Marmot inner event JSONs.
+ *
+ * Marmot MLS group messages are encrypted with per-message ratchet keys that
+ * advance as each message is decrypted. After the ratchet has advanced past
+ * a message, that ciphertext can no longer be decrypted — even by its
+ * original recipient. To make group history survive app restarts we must
+ * persist the *plaintext* inner events at the moment they are first
+ * decrypted, rather than relying on relay redelivery.
+ *
+ * Implementations MUST encrypt all data at rest — the stored blobs contain
+ * the contents of private group conversations.
+ *
+ * Each group's messages are stored independently, keyed by the hex-encoded
+ * Nostr group ID (the `h` tag value from MIP-01).
+ */
+interface MarmotMessageStore {
+    /**
+     * Append a decrypted inner event JSON to the group's persisted message log.
+     * Implementations should be tolerant to duplicate appends.
+     *
+     * @param nostrGroupId hex-encoded Nostr group ID
+     * @param innerEventJson the decrypted inner Nostr event JSON (e.g., kind:9 chat)
+     */
+    suspend fun appendMessage(
+        nostrGroupId: String,
+        innerEventJson: String,
+    )
+
+    /**
+     * Load all persisted inner event JSONs for a group, in append order.
+     *
+     * @param nostrGroupId hex-encoded Nostr group ID
+     * @return list of inner event JSON strings, empty if none
+     */
+    suspend fun loadMessages(nostrGroupId: String): List<String>
+
+    /**
+     * Delete all persisted messages for a group (after leaving).
+     *
+     * @param nostrGroupId hex-encoded Nostr group ID
+     */
+    suspend fun delete(nostrGroupId: String)
+}