fix(pageFilters): clear shift-click anchor on empty selection

[priscilawebdev]

The shift-click range selection in useStagedCompactSelect kept its anchor (lastSelected) pointing at the last clicked option even after the staged selection became empty. The next shift-click would then range-extend from that ghost anchor.

Repro (environment selector on the traces explorer)

With environments [A, B, C, D, E]:

1. Shift-click A — selects [A], anchor = A
2. Shift-click E — range-selects [A, B, C, D, E], anchor = E
3. Shift-click E, D, C, B, A one by one — incrementally deselects the range; selection ends at [], but anchor lingers at A
4. Shift-click C — expected [C], actual [A, B, C]

Fix

In the 'toggle' and 'toggle range' reducer cases, clear lastSelected to null whenever the resulting stagedValue is empty. The next shift-click then falls through the existing lastSelected === null branch, which already treats it as a regular single click (the same path used when the menu first opens).

A stricter check (treat the anchor as invalid whenever it's not in the current selection) was attempted but broke the existing 'deselects range when shift-clicking already selected item' test, which relies on a ctrl-clicked-off anchor still being usable for a range-deselect.

Added a unit test that walks through the exact repro above.

[github-actions]

📊 Type Coverage Diff

✅ No new type safety issues introduced. Coverage: 93.51%