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/GlobalAIAssistantDrawer.tsx

@@ -2,6 +2,7 @@ import { useCallback, useEffect, useRef, useState } from 'react'
 import { Box, Divider, Drawer, IconButton, TextField, Tooltip, Typography } from '@mui/material'
 import { RotateCcw, Send, Sparkles, X } from 'lucide-react'
 import { useLocation } from 'react-router'
+import { useShallow } from 'zustand/react/shallow'
 import { useAssistantStore } from '../../stores/assistantStore'
 import { useSessionStore } from '../../stores/sessionStore'
 import { aiAssistantService } from '../../services/aiAssistantService'
@@ -21,7 +22,7 @@ function resolveWorkspace(pathname: string): WorkspaceType | null {
 
 export function GlobalAIAssistantDrawer() {
   const { isOpen, close, context, setContext, messages, isLoading, error, addMessage, setLoading, setError, clearConversation } =
-    useAssistantStore()
+    useAssistantStore(useShallow(s => s))
 
   const { currentUser } = useSessionStore()
   const location = useLocation()