[tldraw] Bind arrows to group center first, only bind to children in precise mode

## Problem When dragging an arrow onto a group, the arrow currently always binds to individual children inside the group — groups are explicitly filtered out as binding targets. This means there's no way to quickly bind an arrow to "the group as a whole." ## Proposed behavior - **Non-precise mode** (default): When dragging an arrow onto a group, bind to the **center of the group** itself. The group should be the binding target. - **Precise mode** (slow drag / deliberate positioning): When dragging an arrow precisely within a group, bind to the **individual children** of the group as it does today. This gives users a natural two-tier interaction: a quick drag connects to the group, while a precise drag lets you target specific shapes within it. ## Current behavior Groups are excluded from `getShapeAtPoint()` results during arrow binding target resolution, so arrows always pierce through the group and bind to whatever child shape is under the cursor. ## Key code areas - `Editor.g