Improve whiteboard shape selection and arrow snapping

## Summary
Improve whiteboard object interaction so individual shapes are clearly visible and can be selected quickly, and arrows can snap to shapes or objects.

## Requirements
- Render whiteboard shapes as distinct, individually visible objects.
- Support selecting shapes using number keys.
- Add snapping for arrows to shapes or other whiteboard objects.

## Acceptance criteria
- Multiple shapes on the whiteboard remain visually distinguishable as individual objects.
- Pressing the supported number key selects its corresponding shape.
- When creating or adjusting an arrow near a shape/object, its endpoint can snap to that target.
- Snapped arrows remain associated with the intended target when appropriate.

Start by locating the whiteboard shape-rendering, number-key selection, and arrow interaction entry points. Verify how multiple shapes and other whiteboard objects are represented before implementing the interaction changes. Done means shapes remain visually distinct, supported number keys select the matching shape, and arrow endpoints snap to and remain associated with intended targets.

Written by the indexing model from the issue text.