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/App.tsx

@@ -12,7 +12,7 @@ const WORKSPACE_HOME: Record<string, string> = {
 }
 
 function RoleRedirect() {
-  const { currentUser } = useSessionStore()
+  const currentUser = useSessionStore(s => s.currentUser)
   const first = currentUser?.allowedWorkspaces[0] ?? WorkspaceType.SUPPLY
   return <Navigate to={WORKSPACE_HOME[first] ?? '/supply/dashboard'} replace />
 }