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_view_preservation.py

@@ -55,6 +55,14 @@ def test_framing_push_is_not_marked_preserve_view(qtbot) -> None:  # type: ignor
     assert "camera" in scene
     assert "range" in scene["xaxis"]
     assert scene["xaxis"]["autorange"] is False
+    # plotly's camera lives in normalized scene units: the model is centred on
+    # the origin and the eye is only a few units out (not a data-unit distance,
+    # which parked the camera hundreds of units away and shrank the model).
+    camera = scene["camera"]
+    assert camera["center"] == {"x": 0.0, "y": 0.0, "z": 0.0}
+    assert (
+        camera["eye"]["x"] ** 2 + camera["eye"]["y"] ** 2 + camera["eye"]["z"] ** 2
+    ) ** 0.5 < 20.0
 
 
 @pytest.mark.gui
@@ -121,3 +129,67 @@ def test_js_console_messages_reach_the_log(qtbot) -> None:  # type: ignore[no-un
         ("warning", "[web] plot.html:8 careful"),
         ("info", "[web] plotly:9 hello"),
     ]
+
+
+@pytest.mark.gui
+def test_deformed_push_colours_by_scalar_and_ghosts_the_reference(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """The canvas wiring drives the builder's contour + ghost overlays."""
+    import numpy as np
+
+    from otko.services.deformation import DeformationSource
+    from otko.views.canvas3d.model_renderer import RendererMode
+
+    canvas, calls = _canvas_with_captured_js(qtbot)
+    project = load_project(EXAMPLES / "cantilever.osmodel")
+    canvas.set_project(project)
+    ids = [node.id for node in project.nodes]
+    source = DeformationSource(
+        displacements=np.zeros((len(ids), 3)) + np.linspace(0, 1, len(ids))[:, None],
+        node_id_to_row={nid: i for i, nid in enumerate(ids)},
+    )
+
+    canvas.set_mode(RendererMode.DEFORMED, source)
+    canvas.set_show_undeformed(True)
+    calls.clear()
+    canvas.render()
+
+    payload, _preserve = _parse_call(calls[-1])
+    names = {trace.get("name") for trace in payload["data"]}
+    assert "undeformed-reference" in names
+    nodes = next(t for t in payload["data"] if t.get("name") == "nodes")
+    assert nodes["marker"]["showscale"] is True
+    assert nodes["marker"]["colorbar"]["title"]["text"] == "|u|"
+
+
+@pytest.mark.gui
+def test_animation_push_carries_prebuilt_frames(qtbot) -> None:  # type: ignore[no-untyped-def]
+    """``animate`` must hand plotly a frame list, not drive Python re-renders."""
+    import numpy as np
+
+    from otko.services.deformation import DeformationSource
+    from otko.views.canvas_plotly.trace_builder import SceneOptions
+
+    canvas, calls = _canvas_with_captured_js(qtbot)
+    project = load_project(EXAMPLES / "cantilever.osmodel")
+    canvas.set_project(project)
+    canvas._scene = canvas._builder.build(project, SceneOptions())
+
+    ids = [node.id for node in project.nodes]
+    options = [
+        SceneOptions(
+            deformation=DeformationSource(
+                displacements=np.full((len(ids), 3), float(k)),
+                node_id_to_row={nid: i for i, nid in enumerate(ids)},
+            )
+        )
+        for k in (1.0, 2.0, 3.0)
+    ]
+    canvas.animate(options, duration_ms=25, loops=4)
+
+    assert calls and calls[-1].startswith("window.otkoAnimate(")
+    body = calls[-1][len("window.otkoAnimate(") :]
+    literal, end = json.JSONDecoder().raw_decode(body)
+    payload = json.loads(literal)
+    assert len(payload["frames"]) == 3
+    assert payload["frames"][0]["data"], "frames must carry trace data"
+    assert body[end:].startswith(", 25, 4, true")