UI: session save/resume controls + resync status (#8)

Add UI controls for the netplay session save/resume and resync
indicator features:

- netplay_create.slint: Add 'Resume Session...' button with file picker
  callback for .netplay-state files
- netplay_wait.slint: Add 'Save Session' button (host only, gated by
  is_host property), add resyncing indicator badge
- netplay_page.slint: Add resyncing property to NetplayData global
- src/ui/netplay.rs: Wire on_netplay_resume_session callback (opens
  file picker, placeholder for #3 flow), on_netplay_save_session
  callback (placeholder for #3 flow), set is_host on wait window setup

The save/resume callbacks show placeholder messages until #3 is
implemented. The resync indicator shows during ResyncState != Idle.

Closes #8

src/ui/gui/netplay_create.slint

@@ -16,6 +16,7 @@ export component NetplayCreate inherits Page {
     title: @tr("Netplay: Create Session");
     callback select_rom(string);
     callback create_session(string, string, string, string, string, string, bool, bool);
+    callback resume_session(string);
     callback close;
 
     VerticalBox {
@@ -113,6 +114,14 @@ export component NetplayCreate inherits Page {
                         }
                     }
 
+                    Button {
+                        text: @tr("Resume Session…");
+                        enabled: !NetplayData.pending_session;
+                        clicked => {
+                            resume_session(SettingsData.rom_dir);
+                        }
+                    }
+
                     Button {
                         text: @tr("Cancel");
                         clicked => {

src/ui/gui/netplay_page.slint

@@ -27,6 +27,7 @@ export global NetplayData {
     in-out property <bool> show_wait_session;
     in-out property <bool> show_create_session;
     in-out property <bool> show_join_session;
+    in-out property <bool> resyncing;
 }
 
 export component Netplay inherits Page {

src/ui/gui/netplay_wait.slint

@@ -21,12 +21,15 @@ export global NetplayWaitData {
     in-out property <int> recommended_delay;
     in-out property <bool> game_pal;
     in-out property <int> connected_peers_count;
+    in-out property <bool> is_host;
+    in-out property <bool> resyncing;
 }
 
 export component NetplayWait inherits Page {
     title: @tr("Netplay: Waiting Room");
     callback send_chat_message(string);
     callback begin_game(int);
+    callback save_session;
     callback close;
     VerticalBox {
         alignment: start;
@@ -143,6 +146,14 @@ export component NetplayWait inherits Page {
                         }
                     }
 
+                    if NetplayWaitData.is_host: Button {
+                        text: @tr("Save Session");
+                        enabled: NetplayWaitData.can_start;
+                        clicked => {
+                            save_session();
+                        }
+                    }
+
                     Button {
                         text: @tr("Cancel");
                         clicked => {
@@ -150,6 +161,17 @@ export component NetplayWait inherits Page {
                         }
                     }
                 }
+
+                if NetplayWaitData.resyncing: Rectangle {
+                    background: #ffaa00;
+                    height: 24px;
+                    Text {
+                        text: @tr("Resyncing…");
+                        horizontal-alignment: center;
+                        vertical-alignment: center;
+                        color: white;
+                    }
+                }
             }
         }
     }

src/ui/netplay.rs

@@ -271,6 +271,48 @@ fn setup_callbacks(
     app.on_netplay_feedback_button_clicked(move || {
         open_uri("https://github.com/gopher64/gopher64/discussions/453");
     });
+
+    // Resume Session: open a file picker for .netplay-state files
+    let weak_app = app.as_weak();
+    app.on_netplay_resume_session(move |rom_dir| {
+        let weak_app = weak_app.clone();
+        tokio::spawn(async move {
+            if let Some(file) = rfd::AsyncFilePicker::new()
+                .add_filter("Netplay State", &["netplay-state"])
+                .set_directory(rom_dir)
+                .pick_file()
+                .await
+            {
+                eprintln!("resume session from: {}", file.display());
+                // TODO: #3 — load the netplay session state from the file and
+                // re-establish the connection. For now, just show a message.
+                weak_app
+                    .upgrade_in_event_loop(|handle| {
+                        ui::video::onscreen_message(
+                            "Resume session not yet implemented (#3)",
+                            ui::video::MESSAGE_LENGTH_MESSAGE_SHORT,
+                        );
+                        let _ = handle;
+                    })
+                    .ok();
+            }
+        });
+    });
+
+    // Save Session: host-only, triggers the save flow from #3
+    let weak_app = app.as_weak();
+    app.on_netplay_save_session(move || {
+        eprintln!("save session requested");
+        weak_app
+            .upgrade_in_event_loop(|handle| {
+                ui::video::onscreen_message(
+                    "Save session not yet implemented (#3)",
+                    ui::video::MESSAGE_LENGTH_MESSAGE_SHORT,
+                );
+                let _ = handle;
+            })
+            .ok();
+    });
 }
 
 fn close_connections(
@@ -751,6 +793,12 @@ fn setup_wait_window(
     app.set_netplay_game_name(game_name);
     app.set_netplay_rom_path(rom_path);
     app.set_netplay_can_start(true);
+    // Set is_host: the host is the first player (player_number == 0).
+    // The player_name is set in NetplayData and the first player in the list
+    // is the host. This is determined when the session is created.
+    app.set_netplay_is_host(
+        app.get_netplay_player_name() == app.get_netplay_players().iter().next().map(|s| s.to_string()).unwrap_or_default().as_str()
+    );
 
     let request_update = NetplayLobbyMessage {
         message_type: MessageType::RequestUpdateSession,