feat(map): add batch export and adjust layout alignment

- Implement synchronous offscreen wafer map PNG rendering and CSV summary.
- Add "Batch Export" button to file browser sidebar.
- Adjust trend pane bottom spacer to align with About button when visible.
- Fix type check errors and add pytest coverage for batch export.

src/pygui/ISC/Tabs/components/StreamControlPanel.qml

@@ -275,8 +275,15 @@ Rectangle {
                 Layout.fillWidth: true
                 implicitHeight: 28
                 text: "Export"
+                // Nothing to export until a file is loaded or a stream has played
+                enabled: streamController.sensorValues.length > 0
+                opacity: enabled ? 1 : 0.4
 
-                onClicked: exportDialog.open()
+                onClicked: {
+                    // Default into <saveDataDir>/Export (created on demand)
+                    exportDialog.selectedFile = "file://" + deviceController.exportDir() + "/wafer_map.png"
+                    exportDialog.open()
+                }
 
                 background: Rectangle {
                     radius: Theme.radiusSm
@@ -299,6 +306,7 @@ Rectangle {
                 id: exportDialog
                 title: "Export Wafer Map"
                 fileMode: FileDialog.SaveFile
+                defaultSuffix: "png"
                 nameFilters: ["PNG files (*.png)"]
                 onAccepted: {
                     var path = String(selectedFile).replace(/^file:\/\//, "");