chore: Remove dock expand collapse animation (#2815)

## Summary

- remove the spring-driven Dock size transition for expand and collapse
- apply open and collapsed sizes immediately to avoid repeated subtree
layout
- preserve the collapsed bottom tab strip and update motion
documentation

## Test Plan

- `cargo test -p gpui-component dock --lib`
- `cargo check -p gpui-component --all-targets`
- `rustfmt --edition 2024 --check crates/ui/src/dock/dock.rs
crates/base/src/dock/dock_placement.rs`
- `git diff --check`

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