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/AppShell.tsx

@@ -17,8 +17,10 @@ import { WorkspaceType } from '../../domain/enums'
 // ---------------------------------------------------------------------------
 
 export function AppShell() {
-  const { activeWorkspace, sidebarCollapsed, setActiveWorkspace, toggleSidebar } =
-    useLayoutStore()
+  const activeWorkspace = useLayoutStore(s => s.activeWorkspace)
+  const sidebarCollapsed = useLayoutStore(s => s.sidebarCollapsed)
+  const setActiveWorkspace = useLayoutStore(s => s.setActiveWorkspace)
+  const toggleSidebar = useLayoutStore(s => s.toggleSidebar)
   const { currentUser } = useSessionStore()
   const navigate = useNavigate()
   const location = useLocation()