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/supply/ReminderFeed.tsx

@@ -1,5 +1,6 @@
 import { Box, Typography } from '@mui/material'
 import { useReminders } from '../../hooks/useReminders'
+import { useShallow } from 'zustand/react/shallow'
 import { useReminderStore } from '../../stores/reminderStore'
 import { ReminderListRow } from './ReminderListRow'
 import { ReminderCard } from './ReminderCard'
@@ -34,7 +35,13 @@ export function ReminderFeed() {
   const {
     filterType, filterStatus, filterPriority, searchQuery, viewMode,
     setFilterType, setFilterStatus, setFilterPriority, setSearchQuery,
-  } = useReminderStore()
+  } = useReminderStore(useShallow(s => ({
+    filterType: s.filterType, filterStatus: s.filterStatus,
+    filterPriority: s.filterPriority, searchQuery: s.searchQuery,
+    viewMode: s.viewMode, setFilterType: s.setFilterType,
+    setFilterStatus: s.setFilterStatus, setFilterPriority: s.setFilterPriority,
+    setSearchQuery: s.setSearchQuery,
+  })))
 
   if (isLoading) return <ReminderSkeleton />