dock: Animate dock drop placeholders (#2665)

## Summary

- animate Dock drop placeholders from the dragged panel origin to the
active drop target
- smoothly transition between center and split drop regions
- keep the placeholder styling aligned with square Dock panels using the
existing translucent drop-target background
- apply a 150ms ease-out transition and preserve stable drop hit testing

## Test Plan

- `cargo test -p gpui-component dock::tab_panel::tests`
- `cargo test --workspace`
- manually drag panels across center and split drop areas in the Dock
example

Co-authored-by: Codex <codex@openai.com>