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_undeformed_reference.py

@@ -0,0 +1,61 @@
+"""Smoke tests for the undeformed-reference overlay on both backends."""
+
+from __future__ import annotations
+
+import numpy as np
+import pytest
+
+pytest.importorskip("PySide6")
+
+from otko.core import ElasticBeamColumn, ElasticSection, Node, Project
+
+
+def _frame_project() -> Project:
+    b, h = 0.30, 0.50
+    return Project(
+        nodes=[
+            Node(id=1, coords=(0.0, 0.0, 0.0), restraint=(True, True, True, True, True, True)),
+            Node(id=2, coords=(6.0, 0.0, 0.0)),
+        ],
+        sections=[
+            ElasticSection(
+                id=1,
+                name="Rect",
+                E=200e9,
+                A=b * h,
+                Iz=b * h**3 / 12.0,
+                Iy=h * b**3 / 12.0,
+                G=80e9,
+                J=1e-6,
+            )
+        ],
+        elements=[ElasticBeamColumn(id=1, nodes=(1, 2), section_id=1)],
+    )
+
+
+@pytest.mark.gui
+def test_pyvista_undeformed_reference_overlay(qtbot) -> None:  # type: ignore[no-untyped-def]
+    from otko.services.deformation import DeformationSource
+    from otko.views.canvas3d.model_canvas import ModelCanvas
+    from otko.views.canvas3d.model_renderer import RendererMode
+
+    canvas = ModelCanvas()
+    qtbot.addWidget(canvas)
+    canvas.show_project(_frame_project())
+
+    # In MODEL mode there is nothing to reference — the toggle must be a no-op.
+    canvas.set_show_undeformed(True)
+    assert canvas._renderer._undeformed_actor is None
+
+    # A deformed shape gets a ghost wireframe, removed again on toggle off.
+    src = DeformationSource(
+        displacements=np.array([[0.0, 0.0, 0.0], [0.0, 0.0, 0.5]]),
+        node_id_to_row={1: 0, 2: 1},
+    )
+    canvas._renderer.set_mode(RendererMode.DEFORMED, src)
+    canvas.render()
+    canvas.set_show_undeformed(True)
+    assert canvas._renderer._undeformed_actor is not None
+
+    canvas.set_show_undeformed(False)
+    assert canvas._renderer._undeformed_actor is None