fire-planner: UX review pass 1 — fix sidebar/route/PATCH/badges issues

Round-1 fixes from the headless UI review:

Backend
- scenarios PATCH now allows config_json/name/description on cartesian
  scenarios (so users can pin flex_rules + notes that recompute will
  preserve). Core fields (jurisdiction/strategy/etc.) still blocked
  because they're rebuilt on recompute. Existing test updated.

Frontend
- Sidebar Plans switcher: drop the kind=user filter so the switcher
  surfaces all 120 cartesian scenarios that ship out of the box.
- Settings → Milestones now reachable at both /settings (index) and
  /settings/milestones (explicit) — the agent navigated to the latter
  and got a blank page.
- EventGantt background click capture: explicit pointerEvents="all" +
  fillOpacity=0 so click-to-add reliably fires on empty regions
  between bars.
- Plan tab stat badges moved out of the chart card into a dedicated
  row above the fan — previously they overlapped the chart's title,
  legend caption ("p10/p50/p..."), and right-side withdrawal axis.
- Stub tabs (Tax Analytics / Compare / Reports / Estate) and stub
  Settings sub-pages (Dividends / Bonds / Tax / Metrics / Other) get
  a "soon" badge + slate-300 styling so they're clearly placeholders.
- New "Portfolio depleted at this year" pill renders in the badge
  row when the scrubbed year's NW is 0 — previously the badges
  silently went to £0 with no UI cue.
- Test life-event from the smoke run cleaned up from prod DB.

246 pytest pass; mypy/ruff clean; frontend typecheck/test/build green.

frontend/src/App.tsx

@@ -75,6 +75,7 @@ export function App() {
             />
             <Route path="settings" element={<SettingsTab />}>
               <Route index element={<MilestonesSettings />} />
+              <Route path="milestones" element={<MilestonesSettings />} />
               <Route path="rates" element={<RatesSettings />} />
               <Route
                 path="dividends"