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.

pubspec.lock

@@ -241,6 +241,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "7.0.3"
+  dynamic_color:
+    dependency: "direct main"
+    description:
+      name: dynamic_color
+      sha256: "43a5a6679649a7731ab860334a5812f2067c2d9ce6452cf069c5e0c25336c17c"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.8.1"
   fake_async:
     dependency: transitive
     description: