fire-planner: UX review pass 2 — health URL, Progress in shell, Gantt

single-year drag, Settings highlight, Dashboard empty state

Round 2 of agent-driven UX review.

- api status badge was always "unreachable" because client called
  /api/healthz but FastAPI mounts /healthz at root (so k8s probes
  hit it without the SPA prefix). Client now calls /healthz directly.
- Progress page rendered without the shell sidebar/tabs because its
  route was sibling to ScenarioShell; moved it inside as a nested
  route (also drops the redundant "← Plan" breadcrumb since the tab
  bar handles that now).
- EventGantt: single-year (point) events no longer render edge handles
  since the bar is too narrow to distinguish "edge grab" from "middle
  grab" — for points the whole bar moves; resize via the popover.
  Bars wider than 24px keep their edge handles.
- Settings sub-nav: Milestones now points at /settings/milestones
  (consistent active highlight); /settings index redirects there.
- Dashboard "Last 12 months" chart shows an explainer when the
  history has fewer than 2 snapshots instead of an empty axis.
- Stub tabs that are currently active get a slate-300 underline +
  slate-500 text rather than the bold slate-900 — visually honest
  about what's a placeholder.

Frontend typecheck/test/build green.

frontend/src/App.tsx

@@ -2,6 +2,8 @@ import { useQuery } from '@tanstack/react-query';
 import { NavLink, Route, Routes, Link } from 'react-router-dom';
 
 import { api } from '@/api/client';
+import { Navigate } from 'react-router-dom';
+
 import { Compare } from '@/pages/Compare';
 import { Dashboard } from '@/pages/Dashboard';
 import { CashflowTab } from '@/pages/CashflowTab';
@@ -53,9 +55,9 @@ export function App() {
           <Route path="/scenarios" element={<Scenarios />} />
           <Route path="/scenarios/new" element={<ScenarioNew />} />
           <Route path="/scenarios/:id/edit" element={<ScenarioEdit />} />
-          <Route path="/scenarios/:id/progress" element={<ProgressPage />} />
           <Route path="/scenarios/:id" element={<ScenarioShell />}>
             <Route index element={<ScenarioDetail />} />
+            <Route path="progress" element={<ProgressPage />} />
             <Route path="cash-flow" element={<CashflowTab />} />
             <Route
               path="tax-analytics"
@@ -74,7 +76,7 @@ export function App() {
               element={<PlaceholderTab feature="Estate planning" wave={2} />}
             />
             <Route path="settings" element={<SettingsTab />}>
-              <Route index element={<MilestonesSettings />} />
+              <Route index element={<Navigate to="milestones" replace />} />
               <Route path="milestones" element={<MilestonesSettings />} />
               <Route path="rates" element={<RatesSettings />} />
               <Route