feat: Options → Plot Properties dialog with live preview and persistence

Adds a table-driven dialog over RenderStyle.STYLE_FIELDS (colour swatches,
an opacity spin and a label font size), reachable from Options. Edits
preview immediately on every canvas, Cancel restores the style the dialog
opened with, and only OK persists — as JSON under QSettings plot/props,
reloaded on the next launch. Both canvases gain set_style(); the dialog
deliberately exposes plot_style() rather than style() so QWidget.style()
keeps its Qt meaning.

AGENTS.md

@@ -20,6 +20,7 @@ Strict one-way MVVM + services: `views → viewmodels → services → core`.
 - `services/` (`opensees_runner.py`, `persistence.py`, `results.py`, ...): may use core + h5py + openseespy. **No Qt.**
 - `views/`: PySide6/pyvistaqt only. **No direct `import openseespy`** — go through a service.
 - `views/canvas3d/` (**PyVista/VTK**, default) and `views/canvas_plotly/` (plotly.js in a `QWebEngineView`) are two backends for the same central 3D view. Both satisfy the `CanvasBackend` protocol in `views/canvas_base.py`, share one `SelectionState` owned by `MainWindow`, and are swapped live via **Options → Canvas Backend** (persisted in `QSettings` under `canvas/backend`). `MainWindow._activate_canvas` keeps both widgets in a `QStackedWidget` — never destroy a canvas mid-session (VTK leaves dangling make-current callbacks). Backend-specific gaps are declared by `CanvasCapabilities` (e.g. Plotly has no force-diagram overlay or video export yet); gate UI on `canvas.capabilities`, never on the backend name. `canvas_plotly/trace_builder.py` is pure (no Qt, no pyvista) and unit-tested headless.
+- `views/canvas3d/style.py` (`RenderStyle`) is the single source of truth for colours/sizes on **both** backends; renderers must read it rather than hard-coding a colour. The `STYLE_FIELDS` subset is user-editable via **Options → Plot Properties…** (`views/dialogs/plot_properties.py`), previews live through `MainWindow.set_plot_style(..., persist=False)`, and persists as JSON in `QSettings` under `plot/props`.
 - `viewmodels/` bridges core↔Qt (signals, `QUndoStack`); `commands/` holds `QUndoCommand` subclasses.
 - Rules: public functions need type hints + docstring; new domain entities go through Pydantic validation; ops >50 ms run off the GUI thread (`AnalysisWorker` in QThread, cancel via `isInterruptionRequested()`, results cross threads as lightweight `ResultsHandle` to HDF5).