feat: Config tab "Options — Paramètres" with IV gate controls + exit params

- New tab replaces "Journal & Sortie" — consolidates all options-specific settings
- IV Gate section: toggle on/off + 3 sliders (IVR High/Extreme/Skew threshold)
  with live color-coded summary and interdependency guards (high < extreme)
- Exit params section: target, stop-loss, reversal mode + threshold (moved from removed journal tab)
- Backend: GET/PUT /api/config/options-gate endpoint reads/writes 4 config DB keys
- useApi.ts: useOptionsGate + useSaveOptionsGate hooks

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/hooks/useApi.ts

@@ -459,6 +459,26 @@ export const useSaveExitDefaults = () => {
   })
 }
 
+export const useOptionsGate = () =>
+  useQuery({
+    queryKey: ['options-gate-config'],
+    queryFn: () => api.get('/config/options-gate').then(r => r.data),
+    staleTime: 60_000,
+  })
+
+export const useSaveOptionsGate = () => {
+  const qc = useQueryClient()
+  return useMutation({
+    mutationFn: (body: {
+      iv_gate_enabled?: boolean
+      iv_gate_ivr_high?: number
+      iv_gate_ivr_extreme?: number
+      iv_gate_skew_threshold?: number
+    }) => api.put('/config/options-gate', body).then(r => r.data),
+    onSuccess: () => qc.invalidateQueries({ queryKey: ['options-gate-config'] }),
+  })
+}
+
 export const useSimPortfolioRisk = () =>
   useQuery({
     queryKey: ['journal-portfolio-risk'],