fix(search): cancel pending rAF on stop and populate cache synchronously on start

[Copilot]

Two correctness gaps in the keyboard navigation focus cache lifecycle introduced by the MutationObserver/rAF caching layer.

Changes

• stopObservingFocusableElements() — cancel any in-flight requestAnimationFrame before clearing the cache; previously a pending rAF could fire after stop and briefly repopulate focusableElements
• startObservingFocusableElements() — call refreshFocusableElements() synchronously after attaching the observer so the cache is ready before the first keypress; the rAF-scheduled refresh is kept for coalescing subsequent mutation callbacks

function stopObservingFocusableElements() {
  focusableElementsObserver?.disconnect()
  focusableElementsObserver = null
  if (refreshFocusableElementsFrame != null) {
    window.cancelAnimationFrame(refreshFocusableElementsFrame)
    refreshFocusableElementsFrame = null
  }
  focusableElements.value = []
}

// in startObservingFocusableElements(), after observer.observe(...)
refreshFocusableElements()       // immediate — first keypress won't see empty cache
scheduleFocusableElementsRefresh() // rAF — coalesces subsequent mutations

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