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/pages/ScenarioDetail.tsx

@@ -0,0 +1,137 @@
+/**
+ * Scenario detail — params + the latest persisted MC projection.
+ *
+ * Reuses FanChart from the What-If page. If the scenario has no MC run
+ * yet, prompts the user to run /recompute.
+ */
+import { useQuery } from '@tanstack/react-query';
+import { Link, useParams } from 'react-router-dom';
+
+import { api } from '@/api/client';
+import { ApiError } from '@/api/client';
+import { FanChart } from '@/components/FanChart';
+import { gbp, pct } from '@/lib/format';
+
+export function ScenarioDetail() {
+  const params = useParams<{ id: string }>();
+  const id = Number(params.id);
+
+  const scen = useQuery({
+    queryKey: ['scenarios', id],
+    queryFn: () => api.scenarios.get(id),
+    enabled: Number.isFinite(id),
+  });
+  const proj = useQuery({
+    queryKey: ['scenarios', id, 'projection'],
+    queryFn: () => api.scenarios.projection(id),
+    enabled: Number.isFinite(id),
+    retry: (count, err) => {
+      // Don't retry the 404 — it's the "no run yet" empty state.
+      if (err instanceof ApiError && err.status === 404) return false;
+      return count < 2;
+    },
+  });
+
+  if (!Number.isFinite(id)) {
+    return <p className="text-red-700">Invalid scenario id.</p>;
+  }
+
+  if (scen.isLoading) return <p className="text-slate-500">Loading…</p>;
+  if (scen.isError || !scen.data) {
+    return (
+      <div className="rounded-md border border-red-200 bg-red-50 p-4 text-red-800 text-sm">
+        Couldn&apos;t load scenario {id}.
+      </div>
+    );
+  }
+
+  const s = scen.data;
+  const projection = proj.data;
+  const projection404 = proj.isError && proj.error instanceof ApiError && proj.error.status === 404;
+
+  return (
+    <section className="space-y-6">
+      <div className="text-sm">
+        <Link to="/scenarios" className="text-slate-500 hover:text-slate-900">
+          ← Scenarios
+        </Link>
+      </div>
+
+      <header>
+        <h1 className="text-3xl font-semibold tracking-tight">{s.name ?? s.external_id}</h1>
+        <p className="text-sm text-slate-500 mt-1">
+          {s.kind} · {s.jurisdiction} · {s.strategy} · leave UK y{s.leave_uk_year} ·{' '}
+          {s.glide_path} glide · {s.horizon_years}y horizon
+        </p>
+        {s.description && <p className="text-sm text-slate-600 mt-2">{s.description}</p>}
+      </header>
+
+      <div className="grid grid-cols-2 md:grid-cols-4 gap-3">
+        <Stat label="Spending" value={gbp(s.spending_gbp)} />
+        <Stat label="NW seed" value={gbp(s.nw_seed_gbp)} />
+        <Stat label="Annual savings" value={gbp(s.savings_per_year_gbp)} />
+        <Stat label="Horizon" value={`${s.horizon_years}y`} />
+      </div>
+
+      {projection ? (
+        <>
+          <div className="grid grid-cols-2 md:grid-cols-4 gap-3">
+            <Stat label="Success rate" value={pct(projection.success_rate)} accent />
+            <Stat label="Median ending NW" value={gbp(projection.p50_ending_gbp)} />
+            <Stat label="P10 ending" value={gbp(projection.p10_ending_gbp)} />
+            <Stat label="P90 ending" value={gbp(projection.p90_ending_gbp)} />
+            <Stat label="Median lifetime tax" value={gbp(projection.median_lifetime_tax_gbp)} />
+            <Stat
+              label="Median years to ruin"
+              value={projection.median_years_to_ruin ?? 'never'}
+            />
+            <Stat label="MC paths" value={projection.n_paths.toLocaleString()} />
+            <Stat label="Run at" value={new Date(projection.run_at).toLocaleString()} />
+          </div>
+
+          <div className="rounded-lg border border-slate-200 bg-white p-5">
+            <h2 className="text-lg font-semibold mb-2">Portfolio fan</h2>
+            <FanChart yearly={projection.yearly} height={420} showWithdrawal />
+          </div>
+        </>
+      ) : projection404 ? (
+        <div className="rounded-lg border border-slate-200 bg-white p-8 text-center text-slate-500">
+          <p className="font-medium text-slate-700">No projection yet.</p>
+          <p className="text-sm mt-2">
+            Run <code className="px-1">python -m fire_planner recompute-all</code> or{' '}
+            <code>POST /recompute</code> to fill in MC projections for all scenarios.
+          </p>
+        </div>
+      ) : proj.isLoading ? (
+        <p className="text-slate-500">Loading projection…</p>
+      ) : (
+        <div className="rounded-md border border-red-200 bg-red-50 p-4 text-red-800 text-sm">
+          {String((proj.error as Error)?.message ?? proj.error)}
+        </div>
+      )}
+    </section>
+  );
+}
+
+function Stat({
+  label,
+  value,
+  accent,
+}: {
+  label: string;
+  value: string | number;
+  accent?: boolean;
+}) {
+  return (
+    <div className="rounded-md border border-slate-200 bg-white p-3">
+      <div className="text-xs uppercase tracking-wide text-slate-500">{label}</div>
+      <div
+        className={`text-xl font-semibold tabular-nums mt-1 ${
+          accent ? 'text-emerald-700' : ''
+        }`}
+      >
+        {value}
+      </div>
+    </div>
+  );
+}