Fix #55: lock group collapse while customize mode is active

[lucasmendes-design]

Summary

Users could collapse the My Plugins group via its chevron mid-reorder, stranding the customize session with no items visible — surfaced during May 2026 usability testing.

This PR adopts the first option the issue proposes ("stay expanded, chevron disabled"):

• The group toggle <button> carries the native :disabled state for the duration of the customize session — blocks both mouse and keyboard activation.
• The chevron <span> forwards its click via toggle.click(), which is a no-op against a disabled button, so it's neutralised for free.
• A marker class (wp-admin-sidebar-group--reorder-locked) on the locked group drives a cursor: not-allowed + dimmed-opacity rule on the chevron so the affordance reads as inactive.

Closes #55.

Implementation

All changes piggyback on expandGroupsForCustomizing() — the existing function that force-expands reassignable groups on enter and pairs entry-decoration with a restorer. Lock + visual marker are applied alongside the force-expand; the restorer re-enables the toggle before running the existing click-to-restore logic (otherwise the click would be swallowed and the group would be stranded in its forced state).

CSS lives in customizer.css next to the other customize-mode-scoped rules. Opacity dim matches the existing .wp-admin-sidebar-group__customize:disabled treatment.

No behaviour change outside customize mode.

Test plan

• Enter customize mode → My Plugins group expands (existing behaviour preserved).
• Hover the chevron → cursor shows not-allowed, chevron opacity drops to ~0.4.
• Click the chevron → nothing happens; group stays expanded.
• Click the "MY PLUGINS" label area (toggle button) → nothing happens.
• Tab to the group header → the disabled toggle is skipped in tab order; pressing Enter/Space anywhere on it does nothing.
• Click Save → customize mode exits; toggle and chevron return to normal interactivity; pre-customize expand state is restored.
• Click Cancel (with or without unsaved changes) → same as Save: full restoration.
• Refresh mid-customize (beforeunload prompt accepted) → toggle returns to enabled on the next paint (regression check; the disabled state shouldn't survive a reload).

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