fix: drop IFocusableNode from trash and zoom

[mikeharv]

The basics

• I validated my changes

The details

Resolves

Fixes #9792

Proposed Changes

1. Trashcan and zoom controls are no longer IFocusableNodes. They stay ordinary tabindex="0" buttons.
2. FocusableTreeTraverser.findFocusableNodeFor no longer walks from an unregistered tab stop (tabindex >= 0) up to an ancestor node. FocusManager therefore does not steal DOM focus from those controls back to the workspace.
3. New bindings for Enter/Space on trash and zoom controls.
4. Focus rings use :focus-visible instead of blocklyActiveFocus.
5. The trash now follows the blocks canvas in the DOM, which aligns with expectations around tab order.
6. Trash opacity is only applied to the sprite and lid (and not the focus ring)

Reason for Changes

Trash and zoom lived inside the workspace's focusable DOM (svgGroup_) and were also extra tab stops. That made them feel like both separate tab stops and part of the workspace. Tab skipped the workspace if a workspace control had previously been focused, and arrow keys jumped focus from the controls onto blocks.

Dropping IFocusableNode alone is not enough, because the traverser still mapped those elements to the workspace. This would make focusing the elements impossible. Skipping tab stops when traversing parents lets the trash and zoom buttons keep native focus without being Blockly focus nodes.

Test Coverage

• Traverser tests check for expected results with tabindex values of 0 and -1
• Trash and zoom tests assert they are tab stops and are not claimed as workspace focus nodes.

Additional Information

An alternative was considered where the trash and zoom controls remained focusable node but became their own trees. That work was explored here:

• fix: nest trashcan and zoom as separate focus trees #10271