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.

docs/QUICK_GUIDE.md

@@ -66,7 +66,9 @@ correctly.
    periods, frequencies, and participation factors per mode.
 4. **Display → Animate Mode Shape** to view each mode. Use the mode
    selector and the animation controls, and **Export…** if you want a
-   video of the mode shape.
+   video of the mode shape. On the Plotly backend the **Play** button runs
+   the animation inside the viewport (plotly frames); on PyVista it is
+   driven from Python.
 5. Modal results also feed the response-spectrum case: define a response
    spectrum, then run the SRSS or CQC combination and open
    **Display → Show Response Spectrum**.
@@ -77,12 +79,20 @@ After a run, the results/report panel shows a summary for the active case
 (static reactions and forces, modal periods, and so on). Use the display
 actions to inspect the model visually:
 
-- **Display → Show Deformed Shape** — with a scale slider.
+- **Display → Show Deformed Shape** — with a scale slider. The deformed
+  shape is coloured by displacement magnitude with a colourbar, and
+  **Display → Show Undeformed Reference** overlays the original shape for
+  comparison.
 - **Display → Show Force Diagram…** — axial (P), shear (V2/V3), moment
   (M2/M3) diagrams.
 - **Display → Show Pushover Curve**, **Show Time-History**, **Show
   Hysteresis** as applicable.
 
+Nodal and distributed loads are drawn as arrows whose length scales with the
+load magnitude; each load pattern gets its own colour and hovering an arrow
+shows its value. Support symbols show which translation directions are
+restrained.
+
 To hand the analysis to someone else, or to archive exactly what was run,
 export a script:
 
@@ -122,6 +132,23 @@ wrapped in a single macro, so one undo removes the whole step. File
 loads, analysis runs, and display-only changes are not model mutations and
 are not undoable.
 
+## 6. Navigating the 3D view
+
+Both canvas backends (**Options → Canvas Backend**) use the same VTK-style mouse
+bindings, and **Help → Mouse Controls** lists them in the app:
+
+- **Rotate** — left-drag.
+- **Pan** — Shift + left-drag, or middle-drag.
+- **Zoom** — right-drag, or the mouse wheel.
+- **Spin (roll)** — Ctrl + left-drag.
+- **Select** — left-click a node or element; Ctrl+click or Shift+click adds
+  to the selection. A drag moves the view; only a click without movement
+  changes the selection.
+
+View presets: **Ctrl+1** isometric, **Ctrl+2** top (XY), **Ctrl+3** front
+(XZ), **Ctrl+4** right (YZ), **Ctrl+E** zoom extents. The camera keeps Z up,
+so the horizon stays level while you orbit.
+
 ## Where to go next
 
 - [`architecture.md`](architecture.md) — MVVM layering and command order.