fix(mail): infinite re-render in MailSettings + error handling + sync result

- Fix infinite re-render: loadAccounts had selectedAccountId in deps causing
  repeated fetches every render; use functional setState and empty deps
- Fix all error handlers in MailSettings: err?.message || err?.detail instead of String(err)
- Fix sync result handling: backend returns {synced, error?} not {success, synced_count}
- This infinite re-render was flooding the browser with requests, causing
  Network Error for SignatureManager API calls

frontend/src/pages/MailSettings.tsx

@@ -51,14 +51,15 @@ export function MailSettingsPage() {
     try {
       const accs = await fetchAccounts();
       setAccounts(accs);
-      if (accs.length > 0 && !selectedAccountId) {
-        setSelectedAccountId(accs[0].id);
+      if (accs.length > 0) {
+        setSelectedAccountId((prev) => prev || accs[0].id);
       }
-    } catch (err) {
-      toast.error(err instanceof Error ? err.message : String(err));
+    } catch (err: any) {
+      toast.error(err?.message || err?.detail || 'Failed to load accounts');
     }
     setLoading(false);
-  }, [selectedAccountId, toast]);
+  // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, []);
 
   useEffect(() => { loadAccounts(); }, [loadAccounts]);
 
@@ -79,8 +80,8 @@ export function MailSettingsPage() {
         smtp_port: 587,
         password: '',
       });
-    } catch (err) {
-      toast.error(err instanceof Error ? err.message : String(err));
+    } catch (err: any) {
+      toast.error(err?.message || err?.detail || 'Save failed');
     } finally {
       setSaving(false);
     }
@@ -95,8 +96,8 @@ export function MailSettingsPage() {
       } else {
         toast.error(result.message);
       }
-    } catch (err) {
-      toast.error(err instanceof Error ? err.message : String(err));
+    } catch (err: any) {
+      toast.error(err?.message || err?.detail || 'Connection test failed');
     } finally {
       setTesting(null);
     }
@@ -106,9 +107,13 @@ export function MailSettingsPage() {
     setSyncing(accountId);
     try {
       const result = await triggerSync(accountId);
-      toast.success(t('mail.syncSuccess', { count: result.synced_count }));
-    } catch (err) {
-      toast.error(err instanceof Error ? err.message : String(err));
+      if ((result as any)?.error) {
+        toast.error((result as any).error);
+      } else {
+        toast.success(t('mail.syncSuccess'));
+      }
+    } catch (err: any) {
+      toast.error(err?.message || err?.detail || 'Sync failed');
     } finally {
       setSyncing(null);
     }
@@ -120,8 +125,8 @@ export function MailSettingsPage() {
       await deleteAccount(accountId);
       setAccounts((prev) => prev.filter((a) => a.id !== accountId));
       toast.success(t('mail.accountDeleted'));
-    } catch (err) {
-      toast.error(err instanceof Error ? err.message : String(err));
+    } catch (err: any) {
+      toast.error(err?.message || err?.detail || 'Delete failed');
     }
   }, [toast, t]);