feat(graph): pan + zoom on the vault graph view (T-323)

Folds a user zoom (scroll wheel, clamped) and pan (drag) transform into
GraphViewport so the painter and hit-testing move in lockstep — hover and
click keep landing on what's drawn. Zoom scales about the canvas centre; a
fresh graph re-fits and drops the transform.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/builtin/graph/graph_view_test.dart

@@ -1,5 +1,6 @@
 import 'package:clide/builtin/graph/src/graph_view.dart';
 import 'package:clide/src/graph/vault_graph.dart';
+import 'package:flutter/gestures.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -36,4 +37,47 @@ void main() {
     expect(find.byType(GraphView), findsOneWidget);
     await tester.tap(find.byType(GraphView)); // no node, no onOpen — must not throw
   });
+
+  testWidgets('dragging pans the graph; a reverse pan restores it', (tester) async {
+    String? opened;
+    final g = VaultGraph.fromOutlinks({'only.md': const []});
+    await tester.pumpWidget(
+      anchoredHarness(
+        f,
+        SizedBox(
+          width: 400,
+          height: 400,
+          child: GraphView(graph: g, onOpen: (id) => opened = id),
+        ),
+      ),
+    );
+    await tester.pump();
+    final centre = tester.getCenter(find.byType(GraphView));
+    // Pan right — the node leaves the centre, so a centre tap misses. (An equal,
+    // opposite drag cancels the same gesture slop, so the net returns to zero —
+    // robust to the exact slop the arena consumes.)
+    await tester.drag(find.byType(GraphView), const Offset(120, 0));
+    await tester.pump();
+    await tester.tapAt(centre);
+    expect(opened, isNull);
+    await tester.drag(find.byType(GraphView), const Offset(-120, 0));
+    await tester.pump();
+    await tester.tapAt(centre);
+    expect(opened, 'only.md');
+  });
+
+  testWidgets('a scroll signal zooms without throwing', (tester) async {
+    final g = VaultGraph.fromOutlinks({
+      'a.md': const ['b.md'],
+      'b.md': const [],
+    });
+    await tester.pumpWidget(anchoredHarness(f, SizedBox(width: 400, height: 400, child: GraphView(graph: g))));
+    await tester.pump();
+    final centre = tester.getCenter(find.byType(GraphView));
+    final pointer = TestPointer(1, PointerDeviceKind.mouse);
+    await tester.sendEventToBinding(pointer.hover(centre));
+    await tester.sendEventToBinding(pointer.scroll(const Offset(0, -120))); // zoom in
+    await tester.pump();
+    expect(find.byType(GraphView), findsOneWidget);
+  });
 }