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.yaml

@@ -55,6 +55,7 @@ dependencies:
   # Embedded ONNX runtime (local OCR recognition backend)
   flutter_onnxruntime: ^1.8.0
   pdfrx: ^2.4.4
+  dynamic_color: ^1.8.1
 
 # Pin sqlite3 to the exact version whose native binaries are vendored under
 # vendor/sqlite3/ (see hooks block below). Without this, pub re-resolves to the