Implement Ability To Drag Selection Actions Bar

[PauloMFJ]

Overview

• Enable dragging the whole multi-selection by pressing its blue actions bar, mirroring how a single card's actions bar drags the card.
• Extract the shared drag mechanics (pointer capture, screen→page delta, updateShapes, history markers) into a reusable useShapeDragHandle hook, and reduce useCardDragHandle to a thin wrapper over it so the two paths no longer duplicate logic.
• Move Selection into its own selection/ folder and split it into useSelectionPosition (presence + box placement) and useSelectionDragHandle hooks, matching the card base's hook-per-concern structure.
• Updated use of cursor: grabbing to use cursor: all-scroll to closer match tldraw behaviour.

Ticket

[gemini-code-assist]

Code Review

This pull request refactors the drag-handling logic by extracting the core dragging functionality into a reusable useShapeDragHandle hook, which is now utilized by both card and multi-selection drag handles. Additionally, selection positioning logic has been separated into its own hook. The review feedback highlights a TypeScript compilation issue under strict null checks when filtering shapes, recommending flatMap as a type-safe alternative. It also suggests using optional chaining for pointer capture methods to prevent potential runtime errors in testing environments like jsdom.

[beets]

Nice changes!

I've noticed that drag doesn't update the z-index of the card which seems counterintuitive to me. Here's an example (it also appears with a single card drag). Something we can discuss separately and doesn't block this PR.

[beets]

/cc @miss-o-soup for your thoughts on the z-index issue above.