feat: OneNote-style notebooks, text fonts, and page navigation

Add notebook.json containers with multi-member pages, fix PDF text
editing (size/bold/drag/double-tap), index SidecarText in search, and
share keyboard page shortcuts plus a PDF scrubber.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/editor/ui/page_nav_shortcuts.dart

@@ -0,0 +1,57 @@
+// lib/editor/ui/page_nav_shortcuts.dart
+//
+// Shared keyboard page navigation for PDF / slide / office editors.
+// Arrow keys + PageUp/PageDown (+ Home/End when provided).
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+
+class PreviousPageIntent extends Intent {
+  const PreviousPageIntent();
+}
+
+class NextPageIntent extends Intent {
+  const NextPageIntent();
+}
+
+class FirstPageIntent extends Intent {
+  const FirstPageIntent();
+}
+
+class LastPageIntent extends Intent {
+  const LastPageIntent();
+}
+
+/// Wraps [child] so ←/→/PageUp/PageDown(/Home/End) drive page changes.
+Widget pageNavShortcuts({
+  required Widget child,
+  required VoidCallback? onPrevious,
+  required VoidCallback? onNext,
+  VoidCallback? onFirst,
+  VoidCallback? onLast,
+}) {
+  return Focus(
+    autofocus: true,
+    child: CallbackShortcuts(
+      bindings: <ShortcutActivator, VoidCallback>{
+        const SingleActivator(LogicalKeyboardKey.arrowLeft): () =>
+            onPrevious?.call(),
+        const SingleActivator(LogicalKeyboardKey.arrowUp): () =>
+            onPrevious?.call(),
+        const SingleActivator(LogicalKeyboardKey.pageUp): () =>
+            onPrevious?.call(),
+        const SingleActivator(LogicalKeyboardKey.arrowRight): () =>
+            onNext?.call(),
+        const SingleActivator(LogicalKeyboardKey.arrowDown): () =>
+            onNext?.call(),
+        const SingleActivator(LogicalKeyboardKey.pageDown): () =>
+            onNext?.call(),
+        if (onFirst != null)
+          const SingleActivator(LogicalKeyboardKey.home): onFirst,
+        if (onLast != null)
+          const SingleActivator(LogicalKeyboardKey.end): onLast,
+      },
+      child: child,
+    ),
+  );
+}