refactor(frontend): extract DashboardPage into components

Split DashboardPage (578 lines) into focused components:
- DashboardEventCard: event card with chat access
- DashboardMatchCard: match card with status indicators
- MatchRequestCards: incoming/outgoing request cards
- EmptyState: reusable empty state component (in common/)

DashboardPage now 295 lines (-49%)

frontend/src/components/dashboard/index.js

@@ -0,0 +1,3 @@
+export { default as DashboardEventCard } from './DashboardEventCard';
+export { default as DashboardMatchCard } from './DashboardMatchCard';
+export { IncomingRequestCard, OutgoingRequestCard } from './MatchRequestCards';