Refactor home page: extract components, add toast system, debounce search

Major restructuring of the monolithic App.js (2484 lines) into focused,
testable components:

Architecture:
- App.js is now a 189-line routing shell (header, nav, page switching)
- HomePage.js orchestrates all home page state and layout
- Each visual section is its own component with clear props API

Extracted components:
- StatsBar: clickable stat cards that filter by severity
- QuickCVELookup: CVE existence check with inline results
- CVEFilters: search + vendor/severity dropdowns
- CVECard: expandable CVE with vendor entries, docs, tickets
- OpenTicketsPanel: right sidebar open JIRA tickets
- IvantiWorkflowPanel: right sidebar Ivanti workflow status + archive

Extracted modals:
- AddCVEModal: self-contained add form with NVD auto-fill
- EditCVEModal: self-contained edit form with NVD update
- JiraTicketModal: unified add/edit JIRA ticket modal
- ArcherTicketModal: unified add/edit Archer ticket modal

Performance optimizations:
- Debounced search (300ms) via useDebounce hook — eliminates
  redundant API calls on every keystroke
- Memoized groupedCVEs, openTicketCount, criticalCount via useMemo
- Proper state updates (no direct mutation of cveDocuments)
- useCallback on fetch functions to stabilize effect dependencies

UX improvements:
- Toast notification system replaces all alert() calls
- Stat cards are now clickable to filter CVE list by severity
- onKeyDown replaces deprecated onKeyPress
- aria-labels added to interactive elements

Infrastructure:
- ToastContext with auto-dismiss, typed toasts (success/error/warning/info)
- useDebounce custom hook for reuse across the app
- Toast slide-in animation in App.css

frontend/src/App.css

@@ -17,6 +17,18 @@
   }
 }
 
+/* Toast notification slide-in */
+@keyframes toast-slide-in {
+  from {
+    transform: translateX(100%);
+    opacity: 0;
+  }
+  to {
+    transform: translateX(0);
+    opacity: 1;
+  }
+}
+
 :root {
   /* Base Colors - Modern Slate Foundation */
   --intel-darkest: #0F172A;