feat(import): Z-wafer CSV import button with result banner

- importZWaferCsv slot on FileBrowser — converts and adds to list
- SourcePanel: import icon button → FileDialog → importSummary banner
  (auto-dismisses after 5s, loads the converted file into streamController)
- wafer_map_item: showExtremes default → true

src/pygui/backend/visualization/wafer_map_item.py

@@ -124,7 +124,7 @@ class WaferMapItem(QQuickPaintedItem):
         self._margin:   float = 1.0
         self._blend:    float = 0.0
         self._show_labels: bool = True
-        self._show_extremes: bool = False
+        self._show_extremes: bool = True
         self._shape: str = "round"
         self._size: float = 300.0
         self._thickness_data: list[list[float]] = []  # [x_mm, y_mm, t2] triples