feat: persist the geohash nickname so it survives app restarts

The location-chat nickname was in-memory only (ChannelNewMessageViewModel
.geohashNickname), so it reset to empty on process death. It can't be
recovered from relays either: the anonymous per-cell key publishes no kind-0
profile, and kind-20000 messages are ephemeral, so the nickname (a per-message
`n` tag) has no durable home on the network.

Persist it on-device instead, as a single global handle:
- GeohashChatIdentityState gains nickname()/setNickname(), stored in the
  account's encrypted storage next to the per-cell device seed (survives
  restarts, switches with the account).
- The chat screen restores it into the composer on room open, and the
  nickname dialog writes it back on Save.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0172JoMccseEKenyWan6txWV

amethyst/src/main/java/com/vitorpamplona/amethyst/model/GeohashChatIdentityState.kt

@@ -58,6 +58,34 @@ class GeohashChatIdentityState(
 
     @Volatile private var cachedDeviceSeed: ByteArray? = null
 
+    @Volatile private var cachedNickname: String? = null
+
+    /**
+     * The user's display handle for location chats: a single global nickname, persisted per account.
+     * Bitchat carries this as the per-message `["n", …]` tag rather than a kind-0 profile, and kind-20000
+     * messages are ephemeral (relays needn't store them), so the only durable home for it is the device.
+     * Kept in this account's encrypted storage, so it survives restarts and switches with the account.
+     * Empty string means "no nickname set". Reads touch disk on first call — invoke off the main thread.
+     */
+    fun nickname(): String {
+        cachedNickname?.let { return it }
+        synchronized(lock) {
+            cachedNickname?.let { return it }
+            val value = Amethyst.instance.encryptedStorage(signer.pubKey).getString(PREF_NICKNAME, "") ?: ""
+            cachedNickname = value
+            return value
+        }
+    }
+
+    /** Persists the global location-chat nickname (trimmed) for this account. */
+    fun setNickname(value: String) {
+        val trimmed = value.trim()
+        synchronized(lock) {
+            cachedNickname = trimmed
+            Amethyst.instance.encryptedStorage(signer.pubKey).edit { putString(PREF_NICKNAME, trimmed) }
+        }
+    }
+
     /** The Nostr key pair to use inside [geohash]. Derivation is cheap but cached; call off the main thread. */
     fun keyPair(geohash: String): KeyPair =
         synchronized(lock) {
@@ -90,5 +118,6 @@ class GeohashChatIdentityState(
 
     companion object {
         private const val PREF_KEY = "geohash_chat_device_seed"
+        private const val PREF_NICKNAME = "geohash_chat_nickname"
     }
 }

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/geohashChat/GeohashChatScreen.kt

@@ -87,6 +87,8 @@ import com.vitorpamplona.amethyst.ui.screen.loggedIn.chats.publicChannels.send.E
 import com.vitorpamplona.quartz.experimental.bitchat.geohash.GeohashChatEvent
 import com.vitorpamplona.quartz.nip01Core.relay.normalizer.NormalizedRelayUrl
 import com.vitorpamplona.quartz.nip01Core.relay.normalizer.displayUrl
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.withContext
 import com.vitorpamplona.amethyst.commons.icons.symbols.Icon as SymbolIcon
 
 /**
@@ -143,6 +145,13 @@ private fun GeohashChatRoom(
     newMessageModel.init(accountViewModel)
     newMessageModel.load(channel)
 
+    // The nickname lives on-device (the throwaway key has no kind-0 profile, and kind-20000 messages
+    // are ephemeral), so restore the account's saved global handle into the composer on open.
+    LaunchedEffect(newMessageModel) {
+        val saved = withContext(Dispatchers.IO) { accountViewModel.account.geohashIdentity.nickname() }
+        if (saved.isNotBlank()) newMessageModel.geohashNickname = saved
+    }
+
     // Teleport is a fact, not a per-message choice: the app decides whether the sender is physically
     // in this cell. When the device's location is known we compare it to the channel cell (objective);
     // otherwise we fall back to how the user arrived — the map picker passes teleported=true, "near me"
@@ -277,7 +286,7 @@ private fun GeohashIdentityAvatar(
 ) {
     var showNickname by remember { mutableStateOf(false) }
     if (showNickname) {
-        GeohashNicknameDialog(model) { showNickname = false }
+        GeohashNicknameDialog(model, accountViewModel) { showNickname = false }
     }
 
     val postAsSelf = model.geohashPostAsSelf
@@ -342,6 +351,7 @@ private fun GeohashIdentityAvatar(
 @Composable
 private fun GeohashNicknameDialog(
     model: ChannelNewMessageViewModel,
+    accountViewModel: AccountViewModel,
     onDismiss: () -> Unit,
 ) {
     var text by remember { mutableStateOf(model.geohashNickname) }
@@ -349,7 +359,10 @@ private fun GeohashNicknameDialog(
         onDismissRequest = onDismiss,
         confirmButton = {
             TextButton(onClick = {
-                model.geohashNickname = text.trim()
+                val trimmed = text.trim()
+                model.geohashNickname = trimmed
+                // Persist the global handle so it survives app restarts (see GeohashChatIdentityState).
+                accountViewModel.account.geohashIdentity.setNickname(trimmed)
                 onDismiss()
             }) { Text("Save") }
         },