feat: frise chronologique + page Snapshot Externe 4 piliers

- TimelineFrise.tsx : visualisation horizontale scrollable COVID→aujourd'hui
  3 lanes (long/medium/short), événements cliquables, marqueur aujourd'hui/date, axe années
- ExternalSnapshot.tsx (/snapshot) : dashboard 4 piliers du contexte externe
  Pilier 1 Géopolitique : 3 badges L/M/C avec J+ depuis événement + lien /timeline?date=
  Pilier 2 Prix & Marchés : quotes SPY/QQQ/GLD/USO/TLT/UUP live + lien Specialist Desks
  Pilier 3 Régimes Macro : régime dominant + scores scénarios + jauges VIX/DXY/10Y
  Pilier 4 Calendrier Économique : prochains événements + récents, liens Calendar/Institutional
- Timeline.tsx : remplace mini-strip par TimelineFrise, lit ?date= query param
- Sidebar : Snapshot Externe (ScanEye) + Timeline (Layers)

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

frontend/src/components/layout/Sidebar.tsx

@@ -1,7 +1,7 @@
 import { NavLink } from 'react-router-dom'
 import {
   LayoutDashboard, Globe, BarChart2, FlaskConical,
-  History, Calendar, TrendingUp, Zap, DollarSign, Settings, BrainCircuit, Activity, BookOpen, FileBarChart, Brain, ShieldAlert, Microscope, ScrollText, Gauge, GitCompare, Building2, Users, Layers
+  History, Calendar, TrendingUp, Zap, DollarSign, Settings, BrainCircuit, Activity, BookOpen, FileBarChart, Brain, ShieldAlert, Microscope, ScrollText, Gauge, GitCompare, Building2, Users, Layers, ScanEye
 } from 'lucide-react'
 import { useGeoRiskScore, useAiStatus, usePortfolioSummary } from '../../hooks/useApi'
 import clsx from 'clsx'
@@ -27,6 +27,7 @@ const nav = [
   { to: '/calendar', icon: Calendar, label: 'Calendar' },
   { to: '/institutional', icon: Building2, label: 'Inst. Reports' },
   { to: '/specialist-desks', icon: Users, label: 'Specialist Desks' },
+  { to: '/snapshot', icon: ScanEye, label: 'Snapshot Externe' },
   { to: '/timeline', icon: Layers, label: 'Timeline' },
   { to: '/logs', icon: ScrollText, label: 'System Logs' },
   { to: '/config', icon: Settings, label: 'Configuration' },