feat(editor): undo/redo, thumbnails, pen settings

Per the full-refactor plan (P0/P1/P2 modules, all pressure-independent):
- engine/undo_stack: generic snapshot undo/redo (per page in the editor)
- ui/thumbnail_grid: Drawboard-style lazy thumbnail nav sheet (pdfrx)
- input/pen_config + ui/pen_settings_page: configurable side-button /
  eraser-end action mapping, pressure curve, palm sensitivity, finger
  drawing, widths (shared_preferences). Button-action mappings persist
  but consume in the input arbiter later; widths/finger consumed now.
Wired into the live editor (undo/redo + grid + settings buttons). 19
new tests.

test/pen_config_test.dart

@@ -0,0 +1,80 @@
+// test/pen_config_test.dart
+//
+// Unit tests for PenConfig (toJson/fromJson round-trip) and
+// applyPressureCurve. No widgets, no SharedPreferences, no Flutter framework.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:badnote/editor/input/pen_config.dart';
+
+void main() {
+  group('applyPressureCurve', () {
+    test('gamma=1 is identity', () {
+      expect(applyPressureCurve(0.5, 1.0), closeTo(0.5, 1e-9));
+      expect(applyPressureCurve(0.0, 1.0), closeTo(0.0, 1e-9));
+      expect(applyPressureCurve(1.0, 1.0), closeTo(1.0, 1e-9));
+    });
+
+    test('applyPressureCurve(0.5, 2.0) == 0.25', () {
+      expect(applyPressureCurve(0.5, 2.0), closeTo(0.25, 1e-9));
+    });
+
+    test('clamps input below 0', () {
+      expect(applyPressureCurve(-0.5, 1.0), closeTo(0.0, 1e-9));
+    });
+
+    test('clamps input above 1', () {
+      expect(applyPressureCurve(1.5, 1.0), closeTo(1.0, 1e-9));
+    });
+  });
+
+  group('PenConfig toJson / fromJson round-trip', () {
+    test('default config survives round-trip', () {
+      const original = PenConfig();
+      final restored = PenConfig.fromJson(original.toJson());
+      expect(restored, equals(original));
+    });
+
+    test('custom config survives round-trip', () {
+      const original = PenConfig(
+        sideButton: PenButtonAction.undo,
+        eraserEnd: PenButtonAction.pan,
+        pressureGamma: 1.5,
+        palmRejectionMs: 200.0,
+        fingerDrawing: true,
+        penWidth: 0.008,
+        highlighterWidth: 0.03,
+      );
+      final restored = PenConfig.fromJson(original.toJson());
+      expect(restored, equals(original));
+    });
+
+    test('fromJson falls back to defaults for missing keys', () {
+      final restored = PenConfig.fromJson({});
+      expect(restored.sideButton, PenButtonAction.eraser);
+      expect(restored.eraserEnd, PenButtonAction.eraser);
+      expect(restored.pressureGamma, 1.0);
+      expect(restored.palmRejectionMs, 150.0);
+      expect(restored.fingerDrawing, false);
+      expect(restored.penWidth, 0.004);
+      expect(restored.highlighterWidth, 0.02);
+    });
+
+    test('fromJson falls back to defaults for unknown enum names', () {
+      final restored = PenConfig.fromJson({
+        'sideButton': 'unknownAction',
+        'eraserEnd': 'anotherUnknown',
+      });
+      expect(restored.sideButton, PenButtonAction.eraser);
+      expect(restored.eraserEnd, PenButtonAction.eraser);
+    });
+  });
+
+  group('PenConfig copyWith', () {
+    test('copyWith preserves unchanged fields', () {
+      const original = PenConfig(pressureGamma: 2.0);
+      final copy = original.copyWith(fingerDrawing: true);
+      expect(copy.pressureGamma, 2.0);
+      expect(copy.fingerDrawing, true);
+    });
+  });
+}