feat: add side-by-side patent portfolio comparison view

Add /compare route with two-panel layout for comparing company patent
portfolios. Each panel shows patent count, analysis timestamp, and
full LLM narrative. The page is responsive (stacks vertically on
mobile) and supports URL params (?a=nvidia&b=intel) for shareability.

Closes leeworks-agents/SPARC#21

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/Layout.tsx

@@ -1,6 +1,6 @@
 import { Outlet, NavLink, useNavigate } from 'react-router-dom';
 import { useAuth } from '../context/AuthContext';
-import { Search, Layers, BarChart3, Info, Users, LogOut } from 'lucide-react';
+import { Search, Layers, BarChart3, Info, Users, LogOut, GitCompareArrows } from 'lucide-react';
 
 export function Layout() {
   const { user, isAdmin, logout } = useAuth();
@@ -15,6 +15,7 @@ export function Layout() {
     { to: '/analysis', icon: Search, label: 'Analysis' },
     { to: '/batch', icon: Layers, label: 'Batch' },
     { to: '/analytics', icon: BarChart3, label: 'Analytics' },
+    { to: '/compare', icon: GitCompareArrows, label: 'Compare' },
     { to: '/about', icon: Info, label: 'About' },
   ];