feat(editor): pen canvas + Material You UI

Clean-room reimplementation of Saber's input model: we own the gesture
pipeline so pen draws with real pressure (perfect_freehand), two-finger
pinch zooms/pans, and palm is rejected (stylus-priority, 2nd-pointer
cancels stroke). pdfrx renders one page at a time (PdfPageView, no
gestures) under a shared transform; page-based nav. Material You theme
via dynamic_color (system accent + seed fallback) and a floating tonal
tool palette + page pill. Old pdfrx-overlay spike no longer wired.

lib/editor/canvas/pen_stroke.dart

@@ -0,0 +1,47 @@
+// lib/editor/canvas/pen_stroke.dart
+//
+// Pen-first canvas stroke model. Points are stored in NORMALIZED page
+// coordinates ([0,1] x [0,1] relative to the page rectangle) so strokes stay
+// pinned to the page regardless of zoom/pan or the on-screen page size.
+
+import 'package:flutter/foundation.dart';
+
+/// A single captured sample of a stroke.
+///
+/// [x]/[y] are normalized to the page rectangle ([0,1]).
+/// [pressure] is the normalized stylus pressure ([0,1]) or null when the
+/// device reported no usable pressure (perfect_freehand then simulates it).
+@immutable
+class PenPoint {
+  const PenPoint(this.x, this.y, this.pressure);
+
+  final double x;
+  final double y;
+  final double? pressure;
+}
+
+/// Which kind of mark a stroke is.
+enum PenStrokeKind { pen, highlighter }
+
+/// A committed stroke for a single page, in normalized page coordinates.
+@immutable
+class PenStroke {
+  const PenStroke({
+    required this.points,
+    required this.color,
+    required this.width,
+    required this.kind,
+  });
+
+  /// Normalized points (see [PenPoint]).
+  final List<PenPoint> points;
+
+  /// ARGB color value.
+  final int color;
+
+  /// Stroke width expressed as a fraction of the page width, so it scales with
+  /// the page when zoomed. Multiply by the on-screen page width to get pixels.
+  final double width;
+
+  final PenStrokeKind kind;
+}