fix: NodeGraph crashed on render — stale sourceImageB64 reference

The multi-source refactor dropped the global sourceImageB64 prop but
left two references in the eyedropper-sample useEffect, which threw
a ReferenceError on first render and broke the entire Pipeline view.

Replaced the single-canvas eyedropper machinery with a per-Source-node
canvas Map keyed by node id. The effect now iterates every Source in
the graph, decodes its current `nodePreviews[id]` b64 into a hidden
canvas (skipping already-decoded ones via a `_b64` cache key), and
prunes entries for Sources that have been deleted. sampleColorAt()
looks up the canvas matching the clicked Source's id.

Verified: npm run build clean.

src-frontend/src/components/NodeGraph.jsx

@@ -131,19 +131,34 @@ export default function NodeGraph({ graph, onChange, nodePreviews, nodeWidth = 2
   const sampleNodeIdRef = useRef(null)
   sampleNodeIdRef.current = sampleNodeId
 
-  // Offscreen canvas holding decoded source image pixels for eyedropper sampling
-  const sourceSampleCanvasRef = useRef(null)
+  // Per-Source offscreen canvases for eyedropper sampling. Each Source
+  // node's preview b64 is decoded once into a hidden canvas keyed by node
+  // id; sampleColorAt() looks up the canvas matching the clicked Source.
+  const sourceSampleCanvasesRef = useRef(new Map())
   useEffect(() => {
-    if (!sourceImageB64) { sourceSampleCanvasRef.current = null; return }
+    const map = sourceSampleCanvasesRef.current
+    const sourceNodes = (graph?.nodes ?? []).filter(n => n.kind === 'Source')
+    // Drop entries for Sources that no longer exist.
+    for (const id of Array.from(map.keys())) {
+      if (!sourceNodes.some(n => n.id === id)) map.delete(id)
+    }
+    // Decode each Source's current preview b64 into a fresh canvas.
+    for (const n of sourceNodes) {
+      const b64 = nodePreviews?.[n.id]
+      if (!b64) { map.delete(n.id); continue }
+      const existing = map.get(n.id)
+      if (existing && existing._b64 === b64) continue   // already decoded
       const img = new Image()
       img.onload = () => {
         const c = document.createElement('canvas')
         c.width = img.naturalWidth; c.height = img.naturalHeight
         c.getContext('2d').drawImage(img, 0, 0)
-      sourceSampleCanvasRef.current = c
+        c._b64 = b64
+        map.set(n.id, c)
       }
-    img.src = `data:image/jpeg;base64,${sourceImageB64}`
-  }, [sourceImageB64])
+      img.src = `data:image/jpeg;base64,${b64}`
+    }
+  }, [graph, nodePreviews])
 
   // ── Wheel zoom — zoom to cursor, computed from refs to handle rapid scroll ──
   const onWheel = useCallback(e => {
@@ -351,7 +366,7 @@ export default function NodeGraph({ graph, onChange, nodePreviews, nodeWidth = 2
 
   // Sample a pixel from the source image at the given click position (screen coords).
   function sampleColorAt(e, sourceNode) {
-    const c = sourceSampleCanvasRef.current
+    const c = sourceSampleCanvasesRef.current.get(sourceNode.id)
     if (!c) return
     const r   = canvasRef.current.getBoundingClientRect()
     const wx  = (e.clientX - r.left - panRef.current.x) / zoomRef.current