refactor(state): clean layoutStore, add selectors, centralise STALE constants

- layoutStore: remove 4 dead field groups (pinnedPanels, selectedResultId,
  compareTrayVisible, notificationsOpen) — none were read outside the store
- CompareTray: drop dead useLayoutStore side-effect (state was write-only)
- 8 components: replace bare useStore() with explicit selectors / useShallow
  to prevent unnecessary re-renders on unrelated state mutations
- lib/constants: add STALE_MARKET_SIGNALS + STALE_REVIEW_QUEUE (30s each)
- useMarketSignals, useReviewQueue: use global constants instead of
  hook-local magic numbers
- Add STATE_MANAGEMENT.md: decision tree + rules for RQ/Zustand/local/derived

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/layout/RightContextPanel.tsx

@@ -18,7 +18,9 @@ const PANEL_PLACEHOLDERS: Record<RightPanelContentType, string> = {
 }
 
 export function RightContextPanel() {
-  const { isRightPanelOpen, rightPanelContentType, closeRightPanel } = useLayoutStore()
+  const isRightPanelOpen = useLayoutStore(s => s.isRightPanelOpen)
+  const rightPanelContentType = useLayoutStore(s => s.rightPanelContentType)
+  const closeRightPanel = useLayoutStore(s => s.closeRightPanel)
 
   const title = rightPanelContentType ? PANEL_TITLES[rightPanelContentType] : ''
   const placeholder = rightPanelContentType ? PANEL_PLACEHOLDERS[rightPanelContentType] : ''