fix(CDropdown): prevent window listener and popper leaks on unmount

[mrholek]

Problem

1. Window listener leak on unmount. The unmount cleanup effect captured the initial _visible value in a stale closure (useEffect(() => () => { if (_visible) handleHide() }, [])), so it never ran for a dropdown that was opened after mount. Unmounting an open dropdown left click/keyup listeners on window and a live Popper instance.
2. Listener reference mismatch. Listeners were added imperatively in handleShow and removed in handleHide. When any useCallback dependency changed while the dropdown was open (e.g. a new onHide prop or a replaced toggler element), removeEventListener received a different function reference and the original listener stayed attached forever.
3. Spurious onHide on mount. The [visible] sync effect ran handleHide() on first render with the default visible={false}, firing the onHide callback for a dropdown that was never opened.
4. Popper instance leak on re-init. initPopper overwrote _popper.current without destroying the previous instance, leaking its scroll/resize listeners whenever the toggler element was replaced while open.

Changes

• Move keydown/click/keyup listener management to a declarative effect keyed on _visible (the same pattern CModal already uses). React guarantees the cleanup removes exactly the references that were added, including on unmount and when the toggler element changes mid-open.
• Replace the broken unmount effect with one that destroys the Popper instance.
• Guard the [visible] sync effect so handleHide only runs when the dropdown is actually open.
• Memoize usePopper callbacks (useCallback) — they only touch refs, so stable identities are safe; this stops handleHide/context value churn on every parent render.
• Destroy the previous Popper instance inside initPopper before creating a new one.

Tests

• New regression test: replaces the toggler element (via key) while the dropdown is open and asserts the menu keeps position: absolute + data-popper-placement, Popper is re-created against the new toggler node, and the old instance is destroyed. Verified it fails against the previous usePopper.
• Full suite: 127 suites / 349 tests green, no snapshot changes (markup untouched).

Note: the react-hooks/refs lint error on usePopper's popper: _popper.current return pre-exists on main and is left out of scope — fixing it changes the hook's public API.