fix: resolve mangled merge artifacts and add notification summary cards

[praizeD10]

• Fix 11+ mangled files from botched merge (App.tsx, EmptyState.tsx, EventExplorerTable.tsx, EventListPanel.tsx, NotificationTimelineView.tsx, WebhookDeliveryChart.tsx, WebhookFailedTable.tsx, ExportHistorySkeleton.tsx, NotificationSearchPage.tsx, NotificationSearchPage.test.tsx, ExportHistoryPage.tsx, ActivityFeed.tsx, EventExplorerCard.tsx, eventStore.ts, TemplatesPage.tsx, EventExplorerPage.tsx, SyncStatus.test.tsx, notificationExport.test.ts, NotificationSearchBar.test.tsx, ExportHistoryPage.test.tsx)
• Deduplicate EmptyState component: keep canonical API with message/title/icon props
• Remove duplicate App() definitions, restore single functional component
• Fix duplicate contractStatuses prop in EventExplorerCard
• Fix duplicate setStatusFilter in eventStore
• Add 'channels' to Tab type in MobileNavDrawer
• Remove duplicate imports and old API EmptyState calls
• Fix test expectations for sort order and async data loading
• Add notification delivery summary cards with loading skeleton
• Add NotificationSummaryCards, NotificationSummaryPanel, useNotificationSummary
• 24 tests passing, all 236 dashboard tests passing, tsc/eslint/prettier clean

Overview

Related Issue

Closes #499

Changes

Verification

# e.g.
cd dashboard && npm test
cd listener && npm run typecheck && npm test
cd contract/contracts/hello-world && cargo test

How to Test

Checklist

• Branch is up to date with main
• Tests added/updated and all pass locally
• cargo fmt --all run (if Rust changes)
• npm run lint passes (if TypeScript changes)
• Documentation updated if behavior changed