feat: Add configurable arrow styles and curved-arrow editing

[devmobasa]

Summary

Add four arrow styles—Standard, Pointy, Curved, and Double - and wire them through rendering, configuration, the configurator, toolbar controls, sessions, and selection editing.

What changed

• Add configurable arrow styles while keeping Standard as the backward-compatible default.
• Add a draggable bend handle for curved arrows, with Shift snapping.
• Persist each arrow’s style and bend through save/load, undo/redo, duplication, and resizing.
• Add the selection-aware Cycle Arrow Style action:
  • restyles selected editable arrows in one undo step;
  • changes the next-arrow style when no arrows are selected.
• Keep the toolbar style pill focused on choosing the next arrow’s style.
• Update bounds, hit testing, labels, cursors, and selection handles for every style.
• Finalize active bend and spotlight gestures before conflicting actions or toolbar events, preserving coherent undo history.
• Add configuration documentation, configurator controls, keybinding metadata, and regression coverage.