feat(editor): highlight drop targets during connection drag

While the operator is dragging from an output port, every
input port now lights up with a tint so they can see at a
glance where the connection can land. The closest port
within the snap distance gets a larger halo + brighter
border so the operator knows which port will accept the
drop if they release now.

Visual states:
 - input port, no drag:        muted background
 - input port, drag in flight: primary @ 35% alpha
 - input port, closest target: primary fill + 2.5 px border
                                + soft halo glow

The closest-target computation runs once per port per
frame during a drag — O(n) over visible input ports.
Negligible at flow sizes operators care about; revisit if
flows exceed a few dozen steps.

Signed-off-by: flemming-it <sf@flemming.it>

lib/src/widgets/flow_canvas.dart

@@ -483,7 +483,15 @@ class _FlowCanvasState extends State<FlowCanvas> {
     void Function(Offset cursor)? onDropTarget,
   }) {
     final theme = Theme.of(context);
-    const size = 16.0;
+    // During a connection drag, highlight every input port
+    // so the operator sees the landing options; the closest
+    // one gets a larger halo so they know which port the
+    // drop will snap to if they release now.
+    final dragging = _draft != null;
+    final isInputDuringDrag = dragging && !isSource;
+    final isClosest =
+        isInputDuringDrag && _isClosestDropTarget(center);
+    final size = isClosest ? 22.0 : 16.0;
     return Positioned(
       left: center.dx - size / 2,
       top: center.dy - size / 2,
@@ -517,10 +525,27 @@ class _FlowCanvasState extends State<FlowCanvas> {
           child: Container(
             decoration: BoxDecoration(
               shape: BoxShape.circle,
-              color: isSource
+              color: isClosest
                   ? theme.colorScheme.primary
+                  : isSource
+                      ? theme.colorScheme.primary
+                      : isInputDuringDrag
+                          ? theme.colorScheme.primary
+                              .withValues(alpha: 0.35)
                           : theme.colorScheme.surfaceContainerHighest,
-              border: Border.all(color: theme.colorScheme.primary, width: 1.5),
+              border: Border.all(
+                color: theme.colorScheme.primary,
+                width: isClosest ? 2.5 : 1.5,
+              ),
+              boxShadow: isClosest
+                  ? [
+                      BoxShadow(
+                        color: theme.colorScheme.primary
+                            .withValues(alpha: 0.5),
+                        blurRadius: 10,
+                      ),
+                    ]
+                  : null,
             ),
           ),
         ),
@@ -528,6 +553,43 @@ class _FlowCanvasState extends State<FlowCanvas> {
     );
   }
 
+  /// True if this input port is the nearest valid drop
+  /// target to the current draft cursor (within snap
+  /// distance). Used to paint the highlight halo so the
+  /// operator sees which port will accept the connection.
+  bool _isClosestDropTarget(Offset portCenter) {
+    final draft = _draft;
+    if (draft == null) return false;
+    final graph = widget.controller.graph;
+    final layout = widget.controller.layout;
+    final outputsX = _outputsX(graph, layout);
+    const maxDist = 32.0;
+    double bestDist = double.infinity;
+    Offset? best;
+    for (final step in graph.steps) {
+      final keys = step.with_.keys.toList();
+      for (var i = 0; i < keys.length; i++) {
+        final p = _inputPortPosition(step.id, i, layout, outputsX);
+        final d = (draft.cursor - p).distance;
+        if (d < bestDist && d <= maxDist) {
+          bestDist = d;
+          best = p;
+        }
+      }
+    }
+    final outs = graph.outputs.keys.toList();
+    for (var i = 0; i < outs.length; i++) {
+      final p = _inputPortPosition('__outputs__', i, layout, outputsX);
+      final d = (draft.cursor - p).distance;
+      if (d < bestDist && d <= maxDist) {
+        bestDist = d;
+        best = p;
+      }
+    }
+    if (best == null) return false;
+    return (best - portCenter).distance < 0.5;
+  }
+
   void Function(Offset) _dropTargetFor(
     String targetId,
     String targetField,