fix(mail): signature manager error handling - prevent [object Object] display

- Fix error handling in SignatureManager load/save/delete to extract message from ApiError
- Same pattern as sync fix: err?.message || err?.detail instead of String(err)

frontend/src/components/mail/SignatureManager.tsx

@@ -52,8 +52,8 @@ export function SignatureManager() {
         setSignatures(data);
         setError(null);
       })
-      .catch((err) => {
-        setError(err instanceof Error ? err.message : String(err));
+      .catch((err: any) => {
+        setError(err?.message || err?.detail || (typeof err === 'string' ? err : 'Failed to load signatures'));
       })
       .finally(() => setLoading(false));
   }, []);
@@ -90,8 +90,8 @@ export function SignatureManager() {
         toast.success(t('mail.signatureCreated'));
       }
       setShowForm(false);
-    } catch (err) {
-      toast.error(err instanceof Error ? err.message : String(err));
+    } catch (err: any) {
+      toast.error(err?.message || err?.detail || (typeof err === 'string' ? err : 'Save failed'));
     } finally {
       setSaving(false);
     }
@@ -104,8 +104,8 @@ export function SignatureManager() {
       setSignatures((prev) => prev.filter((s) => s.id !== deleteTarget.id));
       toast.success(t('mail.signatureDeleted'));
       setDeleteTarget(null);
-    } catch (err) {
-      toast.error(err instanceof Error ? err.message : String(err));
+    } catch (err: any) {
+      toast.error(err?.message || err?.detail || (typeof err === 'string' ? err : 'Delete failed'));
     }
   }, [deleteTarget, toast, t]);