fix: guard EventSource for jsdom test env, add rate-limits MSW handler

frontend/src/test/mocks/handlers.ts

@@ -118,6 +118,16 @@ export const handlers = [
   // Health
   http.get('/api/health', () => HttpResponse.json({ status: 'ok' })),
 
+  // System
+  http.get('/api/system/rate-limits', () => HttpResponse.json({
+    polygon_global_limit: 45,
+    polygon_source_types: ['market_api', 'news_api'],
+    per_type_limits: { market_api: 20, news_api: 20 },
+    cadences_seconds: { market_api: 300, news_api: 300 },
+    market_api: { rate_per_minute: 20, cadence_seconds: 300, max_tickers_per_cycle: 100, active_sources: 51 },
+    news_api: { rate_per_minute: 20, cadence_seconds: 300, active_sources: 50 },
+  })),
+
   // Macro events and impacts
   http.get('/api/macro/events', () => HttpResponse.json(mockMacroEvents)),
   http.get('/api/macro/events/:id', ({ params }) => {