fix(split): frame scratchpad on existing ink

The pen-first scratchpad opened at identity transform, showing only the
empty top-left corner of the 4000x4000 world — so existing ink (drawn
elsewhere) was off-screen and the pane looked blank ("草稿纸根本没看到").

On first layout, fit the strokes' world bounding box into the pane (padded,
scale clamped 0.15-1.5) so saved ink is immediately visible; an empty
scratchpad falls back to a 1:1 view near the origin.

flutter analyze: 0 issues.

lib/screens/split_view_screen.dart

@@ -60,6 +60,9 @@ class _SplitViewState extends State<SplitViewScreen> {
   /// Pan/zoom transform for the scratchpad world (PenCanvas drives this).
   final TransformationController _scratchTransform = TransformationController();
 
+  /// Set once the initial view has been framed onto existing ink.
+  bool _scratchCentered = false;
+
   Size get _worldSize => Size(_canvasWidth, _canvasHeight);
 
   /// Maps the scratchpad toolbar's [PenTool] to the pen-canvas tool. Shapes and
@@ -471,24 +474,79 @@ class _SplitViewState extends State<SplitViewScreen> {
     // Render the world through the performant PenCanvas: strokes normalized
     // against the current world size; toolbar width is in world pixels, so the
     // pen-canvas fraction is width / worldWidth.
-    return Container(
-      color: Theme.of(context).scaffoldBackgroundColor,
-      child: PenCanvas(
-        pageSize: _worldSize,
-        strokes: penStrokesFromInk(_strokes, _worldSize),
-        transformationController: _scratchTransform,
-        tool: _canvasTool,
-        color: _currentColor,
-        strokeWidth: _currentStrokeWidth / _canvasWidth,
-        // The world is huge, so allow zooming further out to survey it.
-        minScale: 0.1,
-        maxScale: 8.0,
-        onStrokeComplete: _onStrokeComplete,
-        onEraseStroke: _onErase,
-        pageWidget: const ColoredBox(color: Colors.white),
-      ),
+    return LayoutBuilder(
+      builder: (context, constraints) {
+        // On first layout, frame the view so existing ink is actually visible
+        // (otherwise identity shows only the empty top-left corner of the huge
+        // world). Empty scratchpad falls back to a comfortable 1:1 near origin.
+        if (!_scratchCentered) {
+          WidgetsBinding.instance.addPostFrameCallback((_) {
+            if (!mounted) return;
+            _frameScratchpad(
+                Size(constraints.maxWidth, constraints.maxHeight));
+            setState(() => _scratchCentered = true);
+          });
+        }
+        return Container(
+          color: Theme.of(context).scaffoldBackgroundColor,
+          child: PenCanvas(
+            pageSize: _worldSize,
+            strokes: penStrokesFromInk(_strokes, _worldSize),
+            transformationController: _scratchTransform,
+            tool: _canvasTool,
+            color: _currentColor,
+            strokeWidth: _currentStrokeWidth / _canvasWidth,
+            // The world is huge, so allow zooming further out to survey it.
+            minScale: 0.1,
+            maxScale: 8.0,
+            onStrokeComplete: _onStrokeComplete,
+            onEraseStroke: _onErase,
+            pageWidget: const ColoredBox(color: Colors.white),
+          ),
+        );
+      },
     );
   }
+
+  /// Position the scratchpad so existing ink is on-screen. Fits the strokes'
+  /// world bounding box into [pane] (with padding, scale clamped); for an empty
+  /// scratchpad, shows the top-left working area at 1:1.
+  void _frameScratchpad(Size pane) {
+    if (pane.isEmpty) return;
+    if (_strokes.isEmpty) {
+      _scratchTransform.value = Matrix4.identity();
+      return;
+    }
+    double minX = double.infinity, minY = double.infinity;
+    double maxX = -double.infinity, maxY = -double.infinity;
+    for (final s in _strokes) {
+      for (final p in s.points) {
+        if (p.x < minX) minX = p.x;
+        if (p.y < minY) minY = p.y;
+        if (p.x > maxX) maxX = p.x;
+        if (p.y > maxY) maxY = p.y;
+      }
+    }
+    if (minX > maxX) {
+      _scratchTransform.value = Matrix4.identity();
+      return;
+    }
+    const pad = 80.0;
+    final boxW = (maxX - minX) + pad * 2;
+    final boxH = (maxY - minY) + pad * 2;
+    final scale =
+        (pane.width / boxW < pane.height / boxH ? pane.width / boxW : pane.height / boxH)
+            .clamp(0.15, 1.5);
+    final cx = (minX + maxX) / 2;
+    final cy = (minY + maxY) / 2;
+    final tx = pane.width / 2 - scale * cx;
+    final ty = pane.height / 2 - scale * cy;
+    _scratchTransform.value = Matrix4.identity()
+      ..setEntry(0, 0, scale)
+      ..setEntry(1, 1, scale)
+      ..setEntry(2, 2, scale)
+      ..setTranslationRaw(tx, ty, 0);
+  }
 }
 
 /// A marker linking a scratchpad position to a specific PDF page.