settings_ui: Toggle root navbar entries on double-click

[najeira]

Settings navbar root entries can now be expanded or collapsed by double-clicking the row itself.

Single-click navigation is preserved, so clicking a root entry still opens the selected Settings page. The double-click handler only toggles on click_count() == 2, so triple-clicking the row does not toggle it again.

The double-click path reuses the same toggle-and-focus behavior as the disclosure chevron.

Testing:

• cargo fmt --check --package settings_ui
• cargo check -p settings_ui
• cargo test -p settings_ui
• Manually verified that:
  • single-clicking a Settings navbar root entry still opens the selected Settings page
  • double-clicking a Settings navbar root entry expands or collapses it
  • triple-clicking the row does not toggle the entry again
  • clicking the disclosure chevron still expands or collapses the entry

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Improved Settings navigation by allowing root navbar entries to expand or collapse on double-click.

[najeira]

zed-settings-nav-double-click-toggle2.mp4

[ChristopherBiscardi]

We currently do nothing on double-click, so this makes the larger click target available.

• Clicking a category switches to that page
• Clicking the dropdown icon toggles the collapsed status and switches to that page
• With this PR, a single click on the category switches to the page and the double click toggles the collapsed status (in addition to keeping the prior two behaviors)