feat(plotly): opstool-style contour, loads, supports, ghost and animation

Bring the Plotly canvas in line with opstool's visualisation recipes.

Camera / interaction:
- frame the camera in plotly's normalized scene units, using the model
  bounds (grid excluded); a data-unit eye rendered the model as a speck
- use turntable dragmode so Z stays up and the horizon stays level
- align mouse bindings with the PyVista/VTK backend and document them in
  Help → Mouse Controls

Visualisation:
- colour deformed / modal shapes by response with a shared colourbar
- scale load arrows by |F|, tint per load pattern, hover the magnitude,
  and fix cones rendering as oversized fins (sizemode scaled, not absolute)
- draw DOF-accurate support glyphs (ported from opstool; see NOTICE)
- add an undeformed-reference overlay on both canvas backends
- play mode shapes with plotly frame animation in-page

Tests: builder unit tests plus GUI regression tests for gestures, the
deformed push, the undeformed reference and animation payloads.

tests/gui/test_plotly_hover.py

@@ -100,8 +100,15 @@ def test_snap_marker_shows_clears_and_tracks_the_camera(qtbot) -> None:  # type:
     assert float(shown["left"].rstrip("px")) > 0
     assert float(shown["top"].rstrip("px")) > 0
 
-    # Orbiting moves the target on screen without any plotly redraw.
-    _run_js(canvas, qtbot, "Plotly.relayout('plot',{'scene.camera.eye':{x:-9,y:6,z:4}})")
+    # Orbiting moves the target on screen without any plotly redraw. A top
+    # view is used because a point near the scene centre projects to nearly
+    # the same pixel from opposite diagonal views.
+    _run_js(
+        canvas,
+        qtbot,
+        "Plotly.relayout('plot',{'scene.camera.eye':{x:0,y:0,z:40},"
+        "'scene.camera.up':{x:0,y:1,z:0}})",
+    )
     qtbot.wait(600)
     moved = _json_state(_run_js(canvas, qtbot, _MARKER_STATE))
     assert moved["display"] == "block"