Add viewTransform support for external pan/zoom integration

[escamoteur]

Summary

This PR adds viewTransform support to Scribble, enabling seamless integration with external pan/zoom controllers like InteractiveViewer. This solves hit-testing issues that occur when using Transform widgets for zoom.

Problem

When wrapping Scribble in an InteractiveViewer (or similar), the Transform widget affects hit testing. When zoomed out, touches at the edges of the viewport map to coordinates outside Scribble's bounds, causing strokes to fail.

Solution

Instead of transforming the widget, transform coordinates inside Scribble:

1. Scribble stays viewport-sized - Always receives all touches
2. Coordinates transformed - Pointer positions transformed using inverse matrix before storing
3. Rendering transformed - Canvas applies viewTransform when painting
4. Constant line thickness - Lines maintain visual width regardless of zoom level

Changes

ScribbleState

• Added viewTransform: Matrix4? field to both Drawing and Erasing states
• Field is excluded from JSON serialization (runtime-only value)

ScribbleNotifier

• Added setViewTransform(Matrix4? transform) method
• _getPointFromEvent() transforms coordinates using inverse viewTransform
• _erasePoint() also transforms coordinates for consistent erasing
• When viewTransform is set, lines are stored with visual width to maintain constant thickness

Painters

• Apply viewTransform to canvas before rendering
• Use 1/scaleFactor for line width when viewTransform is set
• Lines maintain constant visual thickness at any zoom level

Dependencies

• Added vector_math to dependencies for Matrix4/Vector3 operations

Usage

// Use with a custom pan/zoom controller that only handles two-finger gestures:
TwoFingerInteractiveViewer(
  transformationController: _transformController,
  applyTransformToChild: false, // Don't wrap in Transform
  child: Scribble(notifier: _scribbleNotifier),
);

// Sync transform to Scribble:
_transformController.addListener(() {
  final transform = _transformController.value;
  _scribbleNotifier.setViewTransform(transform);
  _scribbleNotifier.setScaleFactor(transform.getMaxScaleOnAxis());
});

Backwards Compatibility

• When viewTransform is null, behavior is unchanged
• Existing sketches and usage patterns continue to work