frontend: run-now + save-as-scenario + edit form (CRUD complete)

Three small UX wins:

- /scenarios/:id Run now — POSTs /simulate with the scenario's params
  and renders the result in a "Live preview run" card below the
  persisted projection. Removes the recompute-or-wait friction.

- /what-if Save as scenario — appears once a simulation has run.
  Prompts for a name (with a sensible default), POSTs the form values
  to /scenarios as a user scenario, redirects to its detail page.

- /scenarios/:id/edit — PATCH form for user scenarios. Pre-fills from
  current scenario; on save invalidates the scenarios query and
  navigates back to detail. Backend already rejects PATCH on
  cartesian; the UI also hides the Edit button for them.

api.scenarios gained patch(). 7 tests pass, typecheck + build clean.

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

frontend/src/App.tsx

@@ -5,6 +5,7 @@ import { api } from '@/api/client';
 import { Compare } from '@/pages/Compare';
 import { Dashboard } from '@/pages/Dashboard';
 import { ScenarioDetail } from '@/pages/ScenarioDetail';
+import { ScenarioEdit } from '@/pages/ScenarioEdit';
 import { ScenarioNew } from '@/pages/ScenarioNew';
 import { Scenarios } from '@/pages/Scenarios';
 import { WhatIf } from '@/pages/WhatIf';
@@ -44,6 +45,7 @@ export function App() {
           <Route path="/scenarios" element={<Scenarios />} />
           <Route path="/scenarios/new" element={<ScenarioNew />} />
           <Route path="/scenarios/:id" element={<ScenarioDetail />} />
+          <Route path="/scenarios/:id/edit" element={<ScenarioEdit />} />
           <Route path="/compare" element={<Compare />} />
           <Route path="/what-if" element={<WhatIf />} />
         </Routes>