frontend: scenario create + delete (CRUD loop closes)

/scenarios/new — form posts to POST /scenarios with name, description,
jurisdiction, strategy, glide path, leave-UK year, spending, NW seed,
savings, horizon. Required-name validation; on success invalidates the
scenarios query and navigates to the new detail page.

/scenarios/:id — Delete button (user scenarios only; cartesian are
backend-protected). Browser confirm prompt + DELETE /scenarios/{id} +
invalidate + redirect to list.

api.scenarios gains create() and delete(). New ScenarioCreateBody type.

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

frontend/src/App.tsx

@@ -4,6 +4,7 @@ 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 { ScenarioNew } from '@/pages/ScenarioNew';
 import { Scenarios } from '@/pages/Scenarios';
 import { WhatIf } from '@/pages/WhatIf';
 
@@ -40,6 +41,7 @@ export function App() {
         <Routes>
           <Route path="/" element={<Dashboard />} />
           <Route path="/scenarios" element={<Scenarios />} />
+          <Route path="/scenarios/new" element={<ScenarioNew />} />
           <Route path="/scenarios/:id" element={<ScenarioDetail />} />
           <Route path="/what-if" element={<WhatIf />} />
         </Routes>