Polish sidebar transitions and search dismissal

[Andresshamis]

What changed

• dismiss the command bar when the sidebar is pressed or a tab/space is selected
• keep the sidebar action active on the same click
• add a compositor-only emphasis transition while paging between spaces
• cover command-bar dismissal and page-transition behavior with regression tests

Why

The command-bar backdrop deliberately passed sidebar clicks through, while New Tab mode was preserved during space selection and tab selection did not dismiss the bar. This left the search interface open after users returned to the sidebar.

The page treatment also gives outgoing and incoming sidebar spaces a subtle, symmetric transition without publishing scroll geometry through SwiftUI.

Validation

• swift build
• swift test — 465 tests passed
• bash -n script/build_and_run.sh
• git diff --check