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/assistant/GlobalAIAssistantButton.tsx

@@ -3,7 +3,8 @@ import { Sparkles } from 'lucide-react'
 import { useAssistantStore } from '../../stores/assistantStore'
 
 export function GlobalAIAssistantButton() {
-  const { isOpen, open } = useAssistantStore()
+  const isOpen = useAssistantStore(s => s.isOpen)
+  const open = useAssistantStore(s => s.open)
 
   return (
     <Tooltip title="AI Assistent öffnen" placement="left">