feat: causal lab — debug endpoint + error display in UI

frontend/src/pages/CausalLab.tsx

@@ -173,11 +173,21 @@ function TabLibrary() {
   const [saving, setSaving]       = useState(false)
   const [editCoefs, setEditCoefs] = useState<Record<string, number>>({})
   const [loading, setLoading]     = useState(true)
+  const [apiError, setApiError]   = useState<string | null>(null)
+  const [debugInfo, setDebugInfo] = useState<Record<string, unknown> | null>(null)
 
   useEffect(() => {
-    setLoading(true)
+    setLoading(true); setApiError(null)
     api(`/api/causal-lab/templates${catFilter ? `?category=${catFilter}` : ''}`)
-      .then(setTemplates).finally(() => setLoading(false))
+      .then(data => {
+        setTemplates(data)
+        if (data.length === 0) {
+          // Auto-run debug to show diagnostic
+          api('/api/causal-lab/debug').then(setDebugInfo).catch(() => {})
+        }
+      })
+      .catch(e => setApiError(String(e)))
+      .finally(() => setLoading(false))
   }, [catFilter])
 
   function selectTemplate(t: Template) {
@@ -213,6 +223,18 @@ function TabLibrary() {
           {cats.map(c => <option key={c} value={c}>{CAT_LABELS[c] ?? c}</option>)}
         </select>
 
+        {apiError && (
+          <div className="mb-2 p-3 bg-red-900/20 border border-red-700/40 rounded text-xs text-red-300 break-all">
+            <div className="font-bold mb-1">Erreur API</div>
+            {apiError}
+          </div>
+        )}
+        {debugInfo && (
+          <div className="mb-2 p-3 bg-yellow-900/20 border border-yellow-700/40 rounded text-xs text-yellow-200">
+            <div className="font-bold mb-1">Diagnostic</div>
+            <pre className="whitespace-pre-wrap">{JSON.stringify(debugInfo, null, 2)}</pre>
+          </div>
+        )}
         {loading
           ? <div className="text-slate-500 text-xs p-4">Chargement…</div>
           : <div className="space-y-1 overflow-y-auto max-h-[calc(100vh-230px)]">