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/backend/visualization/wafer_map_item.py

@@ -14,7 +14,7 @@ import logging
 import math
 
 import numpy as np
-from PySide6.QtCore import Property, QPoint, Qt, Signal, Slot
+from PySide6.QtCore import Property, QPoint, QRectF, Qt, Signal, Slot
 from PySide6.QtGui import (
     QBrush,
     QColor,
@@ -310,17 +310,42 @@ class WaferMapItem(QQuickPaintedItem):
         Returns:
         True on success, False on failure
         """
-        if not file_path:
+        if not file_path or not self._values:
+            # No file loaded / no stream played yet — only the empty template
+            # would render, which isn't a meaningful export.
             return False
         try:
-            result = self.grabToImage()
-            img = result.image()  # type: ignore[attr-defined]
-            img.save(file_path, "PNG")
-            return True
+            # Render synchronously via paint() into our own image —
+            # grabToImage() resolves on a later render frame, so its result
+            # is null when read immediately (the old silent failure).
+            w, h = int(self.width()), int(self.height())
+            if w <= 0 or h <= 0:
+                return False
+            metrics = self._metrics_text()
+            footer_h = 28 if metrics else 0
+            img = QImage(w, h + footer_h, QImage.Format.Format_ARGB32)
+            img.fill(0)  # transparent background
+            painter = QPainter(img)
+            self.paint(painter)
+            if metrics:
+                footer = QRectF(0, h, w, footer_h)
+                painter.fillRect(footer, QColor("#101014"))
+                painter.setPen(QColor("#CBD5E1"))
+                painter.drawText(footer, Qt.AlignmentFlag.AlignCenter, metrics)
+            painter.end()
+            return bool(img.save(file_path))
         except Exception as e:
             log.error("Export failed: %s", e)
             return False
 
+    def _metrics_text(self) -> str:
+        """One-line metric readout for the export footer; '' when no data."""
+        if not self._values:
+            return ""
+        vs = self._values
+        return (f"Sensors: {len(vs)}   Min: {min(vs):.2f}°C   "
+                f"Max: {max(vs):.2f}°C   Avg: {sum(vs) / len(vs):.2f}°C")
+
     # ── internal ─────────────────────────────────────────────────────────
 
     def _rebuild_thickness(self) -> None: