fix(canvas): persist strokes, pressure, slider

Strokes vanished on pen-up: StaticInkPainter aliased the same mutable
list so shouldRepaint saw no change. Commit/erase now replace the list.
Finger-drawing toggle wins over palm-rejection; pressure surfaces even
when the pen reports no min/max range; pages recenter after a flip; the
keyboard page-jump (unreliable on Windows) is now a drag slider. Also
register the dynamic_color plugin in generated registrants.

lib/editor/canvas/pen_editor_screen.dart

@@ -30,10 +30,16 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
   /// Strokes per page, keyed by 0-based page index (normalized coords).
   final Map<int, List<PenStroke>> _strokesByPage = {};
 
-  /// One shared transform for the current page; reset on page change so each
-  /// page opens fit-to-view.
+  /// One shared transform for the current page; recentred on page change so
+  /// each page opens fit-to-view and centered.
   final TransformationController _transform = TransformationController();
 
+  /// Set when the page must be (re)centered on the next layout pass.
+  bool _needsCenter = true;
+
+  /// Live page value while dragging the page slider (null when not dragging).
+  double? _scrub;
+
   // Tool state.
   CanvasTool _tool = CanvasTool.pen;
   Color _color = Colors.black;
@@ -82,7 +88,13 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
 
   void _commitStroke(PenStroke stroke) {
     setState(() {
-      _strokesByPage.putIfAbsent(_pageIndex, () => <PenStroke>[]).add(stroke);
+      // Replace with a NEW list so StaticInkPainter sees a fresh identity and
+      // actually repaints (mutating in place would alias the old painter's list
+      // and shouldRepaint would see no change → committed strokes vanish).
+      _strokesByPage[_pageIndex] = [
+        ...?_strokesByPage[_pageIndex],
+        stroke,
+      ];
     });
   }
 
@@ -90,7 +102,8 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     setState(() {
       final list = _strokesByPage[_pageIndex];
       if (list != null && index >= 0 && index < list.length) {
-        list.removeAt(index);
+        final next = List<PenStroke>.of(list)..removeAt(index);
+        _strokesByPage[_pageIndex] = next;
       }
     });
   }
@@ -102,40 +115,15 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     if (clamped == _pageIndex) return;
     setState(() {
       _pageIndex = clamped;
-      _transform.value = Matrix4.identity();
+      _needsCenter = true; // recenter the new page on next layout
     });
   }
 
-  Future<void> _promptJumpToPage() async {
-    final doc = _document;
-    if (doc == null) return;
-    final controller = TextEditingController(text: '${_pageIndex + 1}');
-    final result = await showDialog<int>(
-      context: context,
-      builder: (context) => AlertDialog(
-        title: const Text('Go to page'),
-        content: TextField(
-          controller: controller,
-          autofocus: true,
-          keyboardType: TextInputType.number,
-          decoration: InputDecoration(hintText: '1 – ${doc.pages.length}'),
-          onSubmitted: (v) =>
-              Navigator.of(context).pop(int.tryParse(v.trim())),
-        ),
-        actions: [
-          TextButton(
-            onPressed: () => Navigator.of(context).pop(),
-            child: const Text('Cancel'),
-          ),
-          TextButton(
-            onPressed: () =>
-                Navigator.of(context).pop(int.tryParse(controller.text.trim())),
-            child: const Text('Go'),
-          ),
-        ],
-      ),
-    );
-    if (result != null) _goToPage(result - 1);
+  /// Centre [pageSize] within [viewport] via the shared transform.
+  void _centerPage(Size viewport, Size pageSize) {
+    final tx = (viewport.width - pageSize.width) / 2;
+    final ty = (viewport.height - pageSize.height) / 2;
+    _transform.value = Matrix4.identity()..setTranslationRaw(tx, ty, 0);
   }
 
   @override
@@ -206,8 +194,15 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
         final scale = fit < fitH ? fit : fitH;
         final pageSize = Size(page.width * scale, page.height * scale);
 
-        return Center(
-          child: PenCanvas(
+        if (_needsCenter) {
+          WidgetsBinding.instance.addPostFrameCallback((_) {
+            if (!mounted) return;
+            _centerPage(
+                Size(constraints.maxWidth, constraints.maxHeight), pageSize);
+            setState(() => _needsCenter = false);
+          });
+        }
+        return PenCanvas(
             key: ValueKey(_pageIndex),
             pageSize: pageSize,
             strokes: _currentStrokes,
@@ -229,8 +224,7 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
               decoration: const BoxDecoration(color: Colors.white),
               backgroundColor: Colors.white,
             ),
-          ),
-        );
+          );
       },
     );
   }
@@ -305,42 +299,65 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     );
   }
 
-  /// Floating page-control pill: prev / "n / total" / next.
+  /// Floating page-control pill: prev / drag-slider / next. No keyboard input
+  /// (Windows on-screen keyboard is unreliable) — the slider scrubs pages.
   Widget _buildPagePill() {
     final doc = _document!;
     final cs = Theme.of(context).colorScheme;
+    final total = doc.pages.length;
+    final shown = (_scrub ?? (_pageIndex + 1).toDouble()).round();
     return Material(
       color: cs.surfaceContainerHigh,
       elevation: 3,
       borderRadius: BorderRadius.circular(28),
-      child: Padding(
-        padding: const EdgeInsets.symmetric(horizontal: 4, vertical: 2),
-        child: Row(
-          mainAxisSize: MainAxisSize.min,
-          children: [
-            IconButton(
-              tooltip: 'Previous page',
-              icon: const Icon(Icons.chevron_left),
-              onPressed: _pageIndex > 0 ? () => _goToPage(_pageIndex - 1) : null,
-            ),
-            TextButton(
-              onPressed: _promptJumpToPage,
-              child: Text(
-                '${_pageIndex + 1} / ${doc.pages.length}',
-                style: TextStyle(
-                  color: cs.onSurface,
-                  fontWeight: FontWeight.w600,
+      child: ConstrainedBox(
+        constraints: const BoxConstraints(maxWidth: 560),
+        child: Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 4, vertical: 2),
+          child: Row(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              IconButton(
+                tooltip: 'Previous page',
+                icon: const Icon(Icons.chevron_left),
+                onPressed:
+                    _pageIndex > 0 ? () => _goToPage(_pageIndex - 1) : null,
+              ),
+              if (total > 1)
+                Flexible(
+                  child: Slider(
+                    min: 1,
+                    max: total.toDouble(),
+                    value: (_scrub ?? (_pageIndex + 1).toDouble())
+                        .clamp(1, total.toDouble()),
+                    label: '$shown',
+                    divisions: total - 1,
+                    onChanged: (v) => setState(() => _scrub = v),
+                    onChangeEnd: (v) {
+                      setState(() => _scrub = null);
+                      _goToPage(v.round() - 1);
+                    },
+                  ),
+                ),
+              Padding(
+                padding: const EdgeInsets.symmetric(horizontal: 6),
+                child: Text(
+                  '$shown / $total',
+                  style: TextStyle(
+                    color: cs.onSurface,
+                    fontWeight: FontWeight.w600,
+                  ),
                 ),
               ),
-            ),
-            IconButton(
-              tooltip: 'Next page',
-              icon: const Icon(Icons.chevron_right),
-              onPressed: _pageIndex < doc.pages.length - 1
-                  ? () => _goToPage(_pageIndex + 1)
-                  : null,
-            ),
-          ],
+              IconButton(
+                tooltip: 'Next page',
+                icon: const Icon(Icons.chevron_right),
+                onPressed: _pageIndex < total - 1
+                    ? () => _goToPage(_pageIndex + 1)
+                    : null,
+              ),
+            ],
+          ),
         ),
       ),
     );