fix(buzz): don't offer a canvas on a DM unless one already exists

The Canvas icon showed on every Buzz channel including DMs, and its edit FAB
offered to create one there. Buzz itself never does: its canvas entry needs
`hasCanvas || canEditNarrative`, and `canEditNarrative` is
`canManageChannel && selfMember !== null && channelType !== "dm"` — so a DM can
only ever *display* a canvas that already exists, never write one
(ChannelManagementSheet.tsx). We were advertising "start a shared document" in a
two-person conversation, and a canvas created there would have been ours alone
to see.

Mirror both halves: a DM gets the icon only when a canvas is already in cache,
and the canvas screen drops its edit FAB for DMs so what remains is read-only.
Non-DM Buzz channels are unchanged — icon always, editing always.

The has-a-canvas check reads the same BuzzWorkspaceStates registry the canvas
screen renders from and recomposes on `canvasUpdates`, so the icon appears when
the document lands rather than on the next visit.

Verified on emulator-5554: the Buzz DM with Vitor (no canvas) shows only the
overflow, where it used to show Canvas + overflow; `general` on the same relay
still shows Canvas + overflow.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/buzz/BuzzCanvasScreen.kt

@@ -63,6 +63,7 @@ import com.vitorpamplona.amethyst.ui.navigation.topbars.TopBarExtensibleWithBack
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
 import com.vitorpamplona.amethyst.ui.stringRes
 import com.vitorpamplona.quartz.buzz.stream.CanvasEvent
+import com.vitorpamplona.quartz.buzz.workspace.isBuzzDm
 import com.vitorpamplona.quartz.nip01Core.relay.normalizer.RelayUrlNormalizer
 import com.vitorpamplona.quartz.nip29RelayGroups.GroupId
 import kotlinx.coroutines.Dispatchers
@@ -95,14 +96,21 @@ fun BuzzCanvasScreen(
     val canvas = remember(version) { state.canvasNote }
     val content = canvas?.event?.content
 
-    // The channel this canvas belongs to, for the subtitle. Null until its kind-39000 lands, which
-    // only costs the subtitle — the canvas itself is keyed by the raw channel id.
-    val channelName =
+    // The channel this canvas belongs to, for the subtitle and the edit gate. Null until its
+    // kind-39000 lands, which only costs the subtitle — the canvas itself is keyed by the raw id.
+    val channel =
         remember(channelId, relayUrl) {
             RelayUrlNormalizer.normalizeOrNull(relayUrl)?.let { relay ->
-                LocalCache.getRelayGroupChannelIfExists(GroupId(channelId, relay))?.toBestDisplayName()
+                LocalCache.getRelayGroupChannelIfExists(GroupId(channelId, relay))
             }
         }
+    val channelName = channel?.toBestDisplayName()
+
+    // Buzz never lets a DM's canvas be written: its editor's `canEdit` is `canEditNarrative`, which
+    // excludes `channelType === "dm"` outright. A DM reaching this screen at all means a canvas
+    // already exists (see the top bar's gate), so render it read-only rather than offering an edit
+    // that Buzz's own client would never show.
+    val canEdit = channel?.event?.isBuzzDm() != true
 
     var editing by remember { mutableStateOf(false) }
 
@@ -145,8 +153,10 @@ fun BuzzCanvasScreen(
             )
         },
         floatingActionButton = {
-            FloatingActionButton(onClick = { editing = true }) {
-                Icon(symbol = MaterialSymbols.Edit, contentDescription = stringRes(R.string.buzz_canvas_edit))
+            if (canEdit) {
+                FloatingActionButton(onClick = { editing = true }) {
+                    Icon(symbol = MaterialSymbols.Edit, contentDescription = stringRes(R.string.buzz_canvas_edit))
+                }
             }
         },
     ) { padding ->

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chats/publicChannels/relayGroup/RelayGroupTopBar.kt

@@ -37,6 +37,7 @@ import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.State
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
@@ -47,10 +48,12 @@ import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.unit.dp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import com.vitorpamplona.amethyst.R
 import com.vitorpamplona.amethyst.commons.icons.symbols.Icon
 import com.vitorpamplona.amethyst.commons.icons.symbols.MaterialSymbols
 import com.vitorpamplona.amethyst.commons.model.buzz.BuzzRelayDialect
+import com.vitorpamplona.amethyst.commons.model.buzz.BuzzWorkspaceStates
 import com.vitorpamplona.amethyst.commons.model.nip29RelayGroups.RelayGroupChannel
 import com.vitorpamplona.amethyst.commons.model.nip29RelayGroups.RelayGroupMembership
 import com.vitorpamplona.amethyst.model.LocalCache
@@ -170,11 +173,17 @@ fun RelayGroupTopBar(
             }
         },
         actions = {
-            // Buzz workspace canvas (kind 40100): shown on any Buzz-dialect relay so a member can
-            // open the shared markdown doc — or create one when the channel has none yet. The only
-            // affordance that stays an icon: it is this channel's shared document, i.e. content, while
-            // Threads and Share are navigation the reader needs once in a while.
-            if (BuzzRelayDialect.isBuzz(channel.groupId.relayUrl)) {
+            // Buzz canvas (kind 40100): shown on any Buzz-dialect relay so a member can open the
+            // channel's shared markdown doc — or create one when it has none yet. The only affordance
+            // that stays an icon: it is this channel's shared document, i.e. content, while Threads and
+            // Share are navigation the reader needs once in a while.
+            //
+            // Except on a DM, where Buzz itself never offers to *write* one: its canvas entry needs
+            // `hasCanvas || canEditNarrative`, and `canEditNarrative` excludes `channelType === "dm"`
+            // outright. So a DM shows the icon only when a canvas already exists — which is also what
+            // keeps us from advertising "start a shared doc" in a two-person conversation.
+            val hasCanvas by observeBuzzCanvas(channel.groupId.id)
+            if (BuzzRelayDialect.isBuzz(channel.groupId.relayUrl) && (!isDm || hasCanvas)) {
                 IconButton(onClick = { nav.nav(Route.BuzzCanvas(channel.groupId.id, channel.groupId.relayUrl.url)) }) {
                     Icon(
                         symbol = MaterialSymbols.Dashboard,
@@ -372,3 +381,18 @@ private fun RoleBadge(membership: RelayGroupMembership) {
         )
     }
 }
+
+/**
+ * Whether this Buzz channel has a canvas (kind 40100) in cache, recomposing when one lands.
+ *
+ * Only the top bar's DM case needs this: a DM gets the canvas affordance solely when a document
+ * already exists, mirroring Buzz's own `hasCanvas || canEditNarrative` where `canEditNarrative`
+ * excludes DMs. Reads the same [BuzzWorkspaceStates] registry the canvas screen renders from, so the
+ * icon appears the moment the document arrives rather than on the next visit.
+ */
+@Composable
+private fun observeBuzzCanvas(channelId: String): State<Boolean> {
+    val state = remember(channelId) { BuzzWorkspaceStates.getOrCreate(channelId) }
+    val version by state.canvasUpdates.collectAsStateWithLifecycle()
+    return remember(channelId, version) { mutableStateOf(state.canvasNote != null) }
+}