feat: add minimap overview to process diagram

Small overview panel in the bottom-left showing the full diagram
layout with colored node rectangles and an amber viewport indicator.
Click or drag on the minimap to pan the main diagram.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/ProcessDiagram/ProcessDiagram.tsx

@@ -9,6 +9,7 @@ import { DiagramEdge } from './DiagramEdge';
 import { CompoundNode } from './CompoundNode';
 import { ErrorSection } from './ErrorSection';
 import { ZoomControls } from './ZoomControls';
+import { Minimap } from './Minimap';
 import { isCompoundType } from './node-colors';
 import styles from './ProcessDiagram.module.css';
 
@@ -292,6 +293,18 @@ export function ProcessDiagram({
         );
       })()}
 
+      <Minimap
+        sections={sections}
+        totalWidth={totalWidth}
+        totalHeight={totalHeight}
+        scale={zoom.state.scale}
+        translateX={zoom.state.translateX}
+        translateY={zoom.state.translateY}
+        containerWidth={zoom.containerRef.current?.clientWidth ?? 0}
+        containerHeight={zoom.containerRef.current?.clientHeight ?? 0}
+        onPan={zoom.panTo}
+      />
+
       <ZoomControls
         onZoomIn={zoom.zoomIn}
         onZoomOut={zoom.zoomOut}