feat(slide): rebuild PPT annotator on the pen-first canvas

PPT slides now annotate with the single performant inking engine
(PenCanvas) instead of the old ink_canvas, per "all note features on the
pen-first canvas".

- PenSlideScreen: per-slide normalized strokes over each slide image,
  prev/next + slider nav, undo/redo, shared M3 palette, and the pressure
  curve / eraser size+mode / palm rejection from the shared canvas.
- slide_export: pure, tested export geometry. Because strokes are now
  normalized to the page rect, the PDF exporter maps them straight into
  each slide's draw rect — fixing the old exporter's known ink
  misalignment (it guessed live-widget size).
- Route PPT import + open -> PenSlideScreen; delete the dead old
  ppt_annotator_screen. (ink_canvas/annotation_toolbar remain for
  split_view, the last old-canvas screen.)

Tests: slide_export geometry (4). flutter analyze: 0 issues. Suite: 269/269.

lib/screens/home_screen.dart

@@ -10,7 +10,7 @@ import '../editor/canvas/pen_editor_screen.dart';
 import '../services/pdf_service.dart';
 import '../services/pptx_service.dart';
 import '../editor/canvas/pen_note_screen.dart';
-import 'ppt_annotator_screen.dart';
+import '../editor/canvas/pen_slide_screen.dart';
 import 'search_screen.dart';
 import 'settings_screen.dart';
 import 'split_view_screen.dart';
@@ -222,7 +222,7 @@ class HomeScreen extends ConsumerWidget {
     if (context.mounted) {
       Navigator.of(context).push(
         MaterialPageRoute(
-          builder: (_) => PptAnnotatorScreen(
+          builder: (_) => PenSlideScreen(
             filePath: filePath,
             slideImagePaths: slideImages,
             extractedText: extractedText.isEmpty ? null : extractedText,
@@ -527,7 +527,7 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
     );
   }
 
-  // [L2] Route by docType: pdf → PenEditorScreen (pen-first), ppt/pptx → PptAnnotatorScreen
+  // [L2] Route by docType: pdf → PenEditorScreen (pen-first), ppt/pptx → PenSlideScreen
   Future<void> _openDocument(BuildContext context) async {
     final document = widget.document;
     final isPdf = document.docType == 'pdf';
@@ -539,7 +539,7 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
         ),
       );
     } else {
-      // PPT/PPTX: convert to images then push PptAnnotatorScreen
+      // PPT/PPTX: convert to images then push PenSlideScreen
       if (mounted) {
         ScaffoldMessenger.of(this.context).showSnackBar(
           const SnackBar(content: Text('Processing presentation...')),
@@ -557,7 +557,7 @@ class _DocumentTileState extends ConsumerState<_DocumentTile> {
       }
       Navigator.of(this.context).push(
         MaterialPageRoute(
-          builder: (_) => PptAnnotatorScreen(
+          builder: (_) => PenSlideScreen(
             filePath: document.filePath,
             slideImagePaths: slideImages,
             extractedText: extractedText.isEmpty ? null : extractedText,