Add anywidget + plotly to interactive feature for client-side drawing

Add anywidget and plotly>=5.20,<6 to the pixi interactive dep-group so the
prototype notebook can render a custom HTML5/SVG drawing widget. anywidget
is the canonical path for traitlet-based widget sync in VSCode-Remote;
plotly is pinned to 5.x because its 6.0 anywidget-backed FigureWidget does
not relay client-side relayout events back to Python (so layout.shapes
never syncs there).

The Sandbox.ipynb prototype itself lives outside this repo
(/home/.../lustre/projects/spatialdata-plot/), but its current state
implements a working anywidget-based draw canvas: pure client-side SVG
drawing (rectangle drag, polygon click-then-Close-polygon, lasso freehand
drag), shapes pushed back via the `shapes` traitlet, pixel→CS coordinate
mapping that respects matplotlib's origin='upper' image axis, multi-shape
commit per Save, and an explicit "Write last to disk" button for
persistence. Sandbox.anywidget-v0.ipynb is preserved alongside as a
reference snapshot before optimization.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timtreis

pyproject.toml

@@ -62,9 +62,13 @@ doc = [
   "sphinxext-opengraph",
 ]
 interactive = [
+  "anywidget",
   "ipykernel",
   "ipympl",
   "ipywidgets",
+  # pinned to 5.x: plotly 6's anywidget-backed FigureWidget doesn't relay
+  # client-side draw events back to Python, so layout.shapes never syncs.
+  "plotly>=5.20,<6",
   "squidpy",
 ]