refactor: shared components, ConfirmDialog migration, pinned flow fixes

- Create shared Spinner component with sm/md/lg sizes
- Migrate 13 page-level spinners to shared Spinner
- Promote EmptyState to shared component, adopt in MyShares and SessionHistory
- Replace window.confirm with ConfirmDialog in 3 files
- Fix PinnedFlow.tree_type to include maintenance, update emoji display
- Verify sidebar unpin handler already correct (no-op)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/TeamAnalyticsPage.tsx

@@ -1,6 +1,6 @@
 import { useState, useEffect } from 'react'
 import { Link, Navigate } from 'react-router-dom'
-import { BarChart3, Loader2, Users, Target, Clock, TrendingUp } from 'lucide-react'
+import { BarChart3, Users, Target, Clock, TrendingUp } from 'lucide-react'
 import {
   AreaChart,
   Area,
@@ -10,6 +10,7 @@ import {
   Tooltip,
   ResponsiveContainer,
 } from 'recharts'
+import { Spinner } from '@/components/common/Spinner'
 import { analyticsApi } from '@/api'
 import { usePermissions } from '@/hooks/usePermissions'
 import type { TeamAnalyticsResponse, AnalyticsPeriod } from '@/types'
@@ -51,7 +52,7 @@ export default function TeamAnalyticsPage() {
   if (loading) {
     return (
       <div className="flex items-center justify-center min-h-[60vh]">
-        <Loader2 size={32} className="animate-spin text-muted-foreground" />
+        <Spinner />
       </div>
     )
   }