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/screens/home_screen.dart

@@ -68,10 +68,10 @@ class HomeScreen extends ConsumerWidget {
               ).push(MaterialPageRoute(builder: (_) => const SearchScreen()));
             },
           ),
-          // THROWAWAY M1 spike entry (remove with lib/editor/pdf/spike_*).
+          // New pen-first canvas editor (beta).
           IconButton(
-            icon: const Icon(Icons.science_outlined),
-            tooltip: 'M1 Spike (pen/scroll/zoom test)',
+            icon: const Icon(Icons.draw_outlined),
+            tooltip: 'Pen Canvas (beta)',
             onPressed: () => openM1Spike(context),
           ),
         ],