feat: update toolbox search UI

[mikeharv]

The basics

• I validated my changes

The details

Proposed Changes

This updates the UI of the toolbox-search, primarily by moving the search field into the flyout.

The field is now an IPositionable pinned to the flyout's top edge, with a separator at the head of the flyout contents reserving room for it. It is a focusable node of the flyout's workspace, so focusing it does not leave the toolbox or close the flyout, and it persists when the flyout contents is updated (while typing).

• Because the category is basically just a ToolboxCategory again, the createDom_, getId() and getFocusableElement() overrides are gone.
• A separator is created at the top of the flyout sized based on the search field.
• SearchFieldNode makes the input a focusable node of the flyout's workspace
• SearchFlyoutNavigator makes the field the get focused when moving in from the toolbox. In a vertical flyout, it makes it so up/down can cycle between the field and blocks.
• Custom keydown handling is needed for horizontal vs vertical layouts as well for the specific side of the screen where the toolbox appears.

	Before	After
No query
repeat

Reason for Changes

The search field was created in place of the category's label, leading to weird behavior such as not being able to style the category and not being able to select text within the field.

Test Coverage

Five unit tests were added, checking that:

• the separator is at the top of the flyout contents
• appropriate text is shown for when there are no matches
• the field is only visible when the category is selected
• the category's row carries the same classes as an ordinary category
• the remembers the query while the flyout is hidden

Documentation

README updated with details about keyboard navigation.

Additional Information

Claude helped with boiler-plate for new files and some of the initial implementation shape. It also wrote the tests. Every line of generated code was carefully reviewed, if not modified or completely re-written by me.