feat: add comprehensive error handling and logging for vault settings save operations with permission error detection and detailed error messages in UI toast notifications

frontend/app.js

@@ -3456,6 +3456,12 @@
             method: "POST",
             headers: { "Content-Type": "application/json" },
             body: JSON.stringify(settings)
+          }).then(async response => {
+            if (!response.ok) {
+              const errorData = await response.json().catch(() => ({ detail: response.statusText }));
+              throw new Error(errorData.detail || `HTTP ${response.status}`);
+            }
+            return response.json();
           })
         );
       });
@@ -3466,7 +3472,8 @@
       showToast("✓ Paramètres sauvegardés", "success");
     } catch (err) {
       console.error("Failed to save hidden files settings:", err);
-      showToast("Erreur de sauvegarde", "error");
+      const errorMsg = err.message || "Erreur inconnue";
+      showToast(`Erreur: ${errorMsg}`, "error");
     } finally {
       if (btn) { btn.disabled = false; btn.textContent = "💾 Sauvegarder"; }
     }
@@ -3501,7 +3508,8 @@
       await Promise.all([loadVaults(), loadTags()]);
     } catch (err) {
       console.error("Reindex with hidden files error:", err);
-      showToast("Erreur de réindexation", "error");
+      const errorMsg = err.message || "Erreur inconnue";
+      showToast(`Erreur: ${errorMsg}`, "error");
     } finally {
       if (btn) { btn.disabled = false; btn.textContent = "🔄 Réindexer"; }
     }