frontend: compare mode (overlay 2-5 scenarios on one fan chart)

Multi-select on /scenarios — checkbox per row, capped at 5. When ≥2
are picked, a "Compare N" button appears that navigates to
/compare?ids=1,2,3.

/compare page pulls each scenario + its latest projection in
parallel via useQueries. Each scenario gets a distinct hue (5 in
the palette); median lines drawn solid, p10 + p90 dashed at 60%
opacity. Stats table below shows success / p10 / p50 / p90 endings
+ median lifetime tax per scenario, with inline "no run yet" rows
for scenarios missing a projection (404 from /projection treated
as soft state, not error).

7 tests pass (was 5). Bundle still ~470 KB gz.

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

frontend/src/App.tsx

@@ -2,6 +2,7 @@ import { useQuery } from '@tanstack/react-query';
 import { NavLink, Route, Routes, Link } from 'react-router-dom';
 
 import { api } from '@/api/client';
+import { Compare } from '@/pages/Compare';
 import { Dashboard } from '@/pages/Dashboard';
 import { ScenarioDetail } from '@/pages/ScenarioDetail';
 import { ScenarioNew } from '@/pages/ScenarioNew';
@@ -43,6 +44,7 @@ export function App() {
           <Route path="/scenarios" element={<Scenarios />} />
           <Route path="/scenarios/new" element={<ScenarioNew />} />
           <Route path="/scenarios/:id" element={<ScenarioDetail />} />
+          <Route path="/compare" element={<Compare />} />
           <Route path="/what-if" element={<WhatIf />} />
         </Routes>
       </main>