fix(chat): use async save dialog for attachments

The Save-attachment handler called the blocking rfd::FileDialog::save_file()
directly inside iced's update() loop. That blocking dialog spins its own GTK
loop; invoked from within iced's already-running event loop (notably the Linux
xdg-desktop-portal/GTK backend, but also observed wedged on Windows) the dialog
becomes unresponsive — Save/Cancel clicks are never processed.

Convert to rfd::AsyncFileDialog returning a Task, mirroring the existing file
*picker* paths (PickAttachmentFile / PickAvatarFile / PickBackgroundFile) which
already use the async variant. The chosen path's bytes are written when the
future resolves; the status line is reported via a new AttachmentSaved message.
No blocking call remains in the update loop.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -287,6 +287,8 @@ pub enum AppMessage {
     AttachmentFilePicked(Option<(String, Vec<u8>)>),
     /// Save (downloading first if needed) a received attachment to disk.
     SaveAttachment(crate::files::AttachmentId),
+    /// Result of the async save dialog: a status line to show, or None if cancelled.
+    AttachmentSaved(Option<String>),
     /// Fetch (if needed) and start an inline audio attachment.
     PlayAudio(crate::files::AttachmentId),
     PauseAudio,
@@ -1094,12 +1096,12 @@ fn update(state: &mut AppState, message: AppMessage) -> Task<AppMessage> {
                     let needs_save = state.pending_saves.remove(&id);
                     let needs_play = state.pending_plays.remove(&id);
                     state.attachment_data.insert(id, AttachmentState::Ready(data));
-                    if needs_save {
-                        save_attachment_to_disk(state, id);
-                    }
                     if needs_play {
                         play_ready_audio(state, id);
                     }
+                    if needs_save {
+                        return save_attachment_task(state, id);
+                    }
                 }
                 UiEvent::AttachmentFailed { id, error } => {
                     state.pending_saves.remove(&id);
@@ -1663,7 +1665,7 @@ fn update(state: &mut AppState, message: AppMessage) -> Task<AppMessage> {
             // If we already have the bytes, save now; otherwise fetch from the
             // sender and save when AttachmentReady arrives (pending_saves).
             if matches!(state.attachment_data.get(&id), Some(AttachmentState::Ready(_))) {
-                save_attachment_to_disk(state, id);
+                return save_attachment_task(state, id);
             } else if let Some((from, att)) = find_attachment_source(state, id) {
                 if let Ok(eid) = from.parse::<EndpointId>() {
                     state.pending_saves.insert(id);
@@ -1676,6 +1678,11 @@ fn update(state: &mut AppState, message: AppMessage) -> Task<AppMessage> {
                 }
             }
         }
+        AppMessage::AttachmentSaved(msg) => {
+            if let Some(m) = msg {
+                state.status_message = m;
+            }
+        }
         AppMessage::PlayAudio(id) => {
             if matches!(state.attachment_data.get(&id), Some(AttachmentState::Ready(_))) {
                 play_ready_audio(state, id);
@@ -1982,11 +1989,18 @@ fn play_ready_audio(state: &mut AppState, id: crate::files::AttachmentId) {
 
 /// Write a ready attachment's bytes to a user-chosen location via a native save
 /// dialog. The default filename comes from the (already-sanitized) descriptor.
-/// No-op if the bytes aren't ready. Sync dialog: the brief block is acceptable
-/// for a deliberate save action.
-fn save_attachment_to_disk(state: &mut AppState, id: crate::files::AttachmentId) {
+/// Build a Task that opens the native save dialog off the UI thread and writes
+/// the (already-fetched) attachment bytes to the chosen path. No-op task if the
+/// bytes aren't ready.
+///
+/// MUST be async (`rfd::AsyncFileDialog`): the blocking `rfd::FileDialog` spins
+/// its own GTK loop, and invoking it from inside iced's running event loop with
+/// the Linux `xdg-desktop-portal`/GTK backend wedges the dialog — Save/Cancel
+/// stop responding. The file *picker* paths already use the async variant; this
+/// is the one save path that must match.
+fn save_attachment_task(state: &AppState, id: crate::files::AttachmentId) -> Task<AppMessage> {
     let Some(AttachmentState::Ready(data)) = state.attachment_data.get(&id) else {
-        return;
+        return Task::none();
     };
     let data = data.clone();
     let default_name = state
@@ -1999,16 +2013,23 @@ fn save_attachment_to_disk(state: &mut AppState, id: crate::files::AttachmentId)
                 .map(|a| a.name.clone())
         })
         .unwrap_or_else(|| "download".to_string());
-    if let Some(path) = rfd::FileDialog::new()
+    Task::perform(
+        async move {
+            let handle = rfd::AsyncFileDialog::new()
                 .set_file_name(default_name)
                 .set_title("Save attachment")
                 .save_file()
-    {
-        match std::fs::write(&path, &data) {
-            Ok(()) => state.status_message = format!("Saved {}", path.display()),
-            Err(e) => state.status_message = format!("Save failed: {e}"),
-        }
+                .await;
+            match handle {
+                Some(h) => match std::fs::write(h.path(), &data) {
+                    Ok(()) => Some(format!("Saved {}", h.path().display())),
+                    Err(e) => Some(format!("Save failed: {e}")),
+                },
+                None => None,
             }
+        },
+        AppMessage::AttachmentSaved,
+    )
 }
 
 fn horizontal_space() -> iced::widget::Space {