frontend: scenarios list + detail pages with persisted fan chart

/scenarios — table of all scenarios with filter chips (all/cartesian/
user). Cartesian scenarios get a neutral badge; user-defined get an
emerald accent. Empty-state nudges the user to run /recompute.

/scenarios/:id — params summary + the latest persisted MC projection.
Reuses FanChart so chart code is shared with /what-if. 404 on the
projection endpoint is treated as "no run yet" (don't retry); other
errors surface normally.

Nav grew a Scenarios tab. typecheck + 5 tests + build pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -3,6 +3,8 @@ import { NavLink, Route, Routes, Link } from 'react-router-dom';
 
 import { api } from '@/api/client';
 import { Dashboard } from '@/pages/Dashboard';
+import { ScenarioDetail } from '@/pages/ScenarioDetail';
+import { Scenarios } from '@/pages/Scenarios';
 import { WhatIf } from '@/pages/WhatIf';
 
 export function App() {
@@ -18,6 +20,7 @@ export function App() {
             </Link>
             <nav className="flex gap-4 text-sm">
               <NavTab to="/">Dashboard</NavTab>
+              <NavTab to="/scenarios">Scenarios</NavTab>
               <NavTab to="/what-if">What if</NavTab>
             </nav>
           </div>
@@ -36,6 +39,8 @@ export function App() {
       <main className="flex-1 max-w-7xl w-full mx-auto px-6 py-8">
         <Routes>
           <Route path="/" element={<Dashboard />} />
+          <Route path="/scenarios" element={<Scenarios />} />
+          <Route path="/scenarios/:id" element={<ScenarioDetail />} />
           <Route path="/what-if" element={<WhatIf />} />
         </Routes>
       </main>