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.

NOTICE

@@ -51,8 +51,11 @@ The canvas element colour palette in `src/otko/views/canvas3d/style.py`
 (per-family element colours and the diverging response colour scale) and the
 resulting diagram colouring in `views/canvas3d/diagram_renderer.py` were
 adapted from the `opstool` project, which is distributed under the **GNU
-General Public License v3.0**. opstool is Copyright © Yexiang Yan and
-contributors.
+General Public License v3.0**. The support (boundary-condition) glyph
+geometry in `views/canvas_plotly/trace_builder.py` (`_support_loops`) is
+ported and modified from opstool's `_get_bc_points_3d` / `_get_bc_points_2d`,
+and the plotly response-colour and load-pattern presentation follows
+opstool's plotly recipes. opstool is Copyright © Yexiang Yan and contributors.
 
 In accordance with GPLv3 §5(a)/(b) this notice records that the material was
 modified and adapted for OTKO. Combining the GPLv3-covered material with