sessions: keep chevron and home button always visible in customizations header (#313507)

* sessions: keep chevron and home button always visible in customizations header

Move the collapse chevron from inside the 'Customizations' link button
to a dedicated container at the far right of the header, with the home
overview button to its left. Both are now always visible (no longer
hover-gated), making collapse/expand and overview entry points
discoverable at a glance.

- Chevron is a flex child of the header (no more absolute positioning)
- Clicking the chevron container toggles collapse, same as clicking the
  header label
- Header total count still surfaces only when collapsed

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address review: scope chevron toggle styling and improve a11y

- Restore .customization-link-counts absolute positioning (it is also
  used inside per-item link buttons via customizationsToolbar.contribution.ts;
  the inline-flex change was breaking the per-item count alignment).
- Use a new dedicated .ai-customization-collapse-toggle class for the
  header chevron container so its styling does not collide with the
  per-item counts class.
- Render the chevron container as a real <button> with aria-label and
  title, and wire it up via Gesture.addTarget for touch (Tap) support
  in addition to the click  matches the pattern used inhandler
  agentHostModePicker.ts.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>