Add a FreeCAD-style navigation cube with rotation arrows

[markomarkovic]

This PR adds an interactive navigation cube to the upper-right corner of the 3D viewport, similar to FreeCAD's, plus two curved arrow buttons that roll the view in 45° steps around the screen-normal axis. It addresses the common problem of losing orientation with the Turntable/Trackball controls by providing a persistent orientation reference with one-click reorientation.

Features

• The cube is OCCT's native AIS_ViewCube, displayed as a transform-persistent object in the existing view — it follows every rotation source (both orbit methods, keyboard shortcuts, toolbar view actions) with no camera-sync code, keeps a constant screen size, and is unaffected by zoom
• Clicking a face, edge or corner starts an eased camera animation to that orientation; zoom, center, distance and projection type are preserved (OCCT's default fit-on-click is undone by re-framing the animation's target camera)
• Cube faces/edges/corners and the arrows highlight on hover; hover detection is confined to the cube's screen region, so the rest of the viewport keeps its existing no-hover behavior, and selection semantics are unchanged
• The rotation arrows are screen-fixed 2D overlays (they don't rotate with the camera) that roll the camera as a pure animated rotation
• New "Show navigation cube" preference (default on); the overlay objects are excluded from fit()
• The cube's FRONT/BACK labels match the toolbar's Front/Back actions. Note this is the opposite of OCCT's default (which labels the −Y face FRONT, as FreeCAD does); since CQ-editor's front_view looks from +Y, consistency with the existing toolbar was chosen over OCCT's convention. As a consequence the default Iso view faces the side labeled BACK — happy to flip the convention instead if that's preferred.

Implementation notes

• A few non-obvious OCCT behaviors are handled and covered by tests: HandleClick requires ViewAnimation().SetView() (segfault otherwise) and a LastActiveView; the finishing animation frame only invalidates the view (an explicit redraw is needed); clicks landing mid-animation must settle the running animation first, or OCCT's repeated-click detection (which snaps the camera up vector to its canonical value) compares against a partially rotated camera
• 14 new tests in tests/test_navigation_cube.py (pytest-qt), covering display, reorientation with zoom/center/pan preservation, animation edge cases, hover, the preference, and fit() exclusion

Disclosure

The code in this PR was written by an AI assistant (Claude, by Anthropic) working under my direction: I specified the feature, made the design decisions, tested the result interactively, and reviewed the changes. Happy to adjust anything that doesn't fit the project's conventions.