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/main.dart

@@ -1,3 +1,4 @@
+import 'package:dynamic_color/dynamic_color.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:google_fonts/google_fonts.dart';
@@ -34,30 +35,36 @@ class BadNoteApp extends ConsumerWidget {
   Widget build(BuildContext context, WidgetRef ref) {
     final settings = ref.watch(settingsProvider);
 
-    return MaterialApp(
-      title: 'BadNote',
-      themeMode: settings.themeMode,
-      theme: ThemeData(
-        colorScheme: ColorScheme.fromSeed(
-          seedColor: settings.colorSchemeSeed,
-          brightness: Brightness.light,
-        ),
-        textTheme: GoogleFonts.interTextTheme(
-          ThemeData(brightness: Brightness.light).textTheme,
-        ),
-        useMaterial3: true,
-      ),
-      darkTheme: ThemeData(
-        colorScheme: ColorScheme.fromSeed(
-          seedColor: settings.colorSchemeSeed,
-          brightness: Brightness.dark,
-        ),
-        textTheme: GoogleFonts.interTextTheme(
-          ThemeData(brightness: Brightness.dark).textTheme,
-        ),
-        useMaterial3: true,
-      ),
-      home: const HomeScreen(),
+    // Material You: prefer the OS dynamic color (Windows/Android system accent);
+    // fall back to the user's seed color when the platform provides none.
+    return DynamicColorBuilder(
+      builder: (lightDynamic, darkDynamic) {
+        final lightScheme = lightDynamic?.harmonized() ??
+            ColorScheme.fromSeed(
+              seedColor: settings.colorSchemeSeed,
+              brightness: Brightness.light,
+            );
+        final darkScheme = darkDynamic?.harmonized() ??
+            ColorScheme.fromSeed(
+              seedColor: settings.colorSchemeSeed,
+              brightness: Brightness.dark,
+            );
+        return MaterialApp(
+          title: 'BadNote',
+          themeMode: settings.themeMode,
+          theme: _theme(lightScheme),
+          darkTheme: _theme(darkScheme),
+          home: const HomeScreen(),
+        );
+      },
     );
   }
+
+  ThemeData _theme(ColorScheme scheme) => ThemeData(
+        colorScheme: scheme,
+        useMaterial3: true,
+        textTheme: GoogleFonts.interTextTheme(
+          ThemeData(brightness: scheme.brightness).textTheme,
+        ),
+      );
 }