feat: multi-tab instrument analysis

Each /instruments/:id navigation opens a persistent tab in the TabBar.
Multiple instruments can be open simultaneously — state is preserved when switching.

- TabsContext: adds instrumentIds[], openInstrument(), closeInstrument()
- TabBar: renders instrument tabs (teal, monospaced, TrendingUp icon) after static tabs, separated by a divider
- InstrumentDashboard: accepts instrumentIdProp so keep-alive instances use the right id regardless of URL
- App: InstrumentRoute registers the tab on navigation; InstrumentKeepAlive mounts one dashboard per open instrument; NormalRoutes hides on instrument paths (hidden div, Routes still fires)

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

frontend/src/pages/InstrumentDashboard.tsx

@@ -1202,8 +1202,8 @@ const PERIODS = [
   { key: '5y',  label: '5Y'  },
 ]
 
-export default function InstrumentDashboard() {
-  const { id = 'SPY' } = useParams<{ id: string }>()
+export default function InstrumentDashboard({ instrumentIdProp }: { instrumentIdProp?: string } = {}) {
+  const { id: paramId = 'EURUSD' } = useParams<{ id: string }>()
   const navigate = useNavigate()
   const [period, setPeriod]           = useState('1y')
   const [chartStyle, setChartStyle]   = useState<'candles' | 'line'>('candles')
@@ -1226,7 +1226,8 @@ export default function InstrumentDashboard() {
   const chartCanvasLeftRef = useRef<number>(0)
   const [chartReady, setChartReady] = useState(0)   // bump to trigger frise re-render
 
-  const instrumentId = id.toUpperCase()
+  // instrumentIdProp is set when mounted as a keep-alive tab (so URL params don't bleed across instances)
+  const instrumentId = (instrumentIdProp ?? paramId).toUpperCase()
 
   useEffect(() => {
     api.get('/instruments').then(r => setInstruments(r.data)).catch(() => {})