fix: prevent [object Object] rendering in 25 frontend components

Replace direct {error} JSX rendering with typeof check + .message fallback.
When error is an object (not a string), React showed [object Object].
Now renders error.message or fallback string.

frontend/src/components/mail/LabelManager.tsx

@@ -103,7 +103,7 @@ export function LabelManager() {
   if (error) {
     return (
       <div className="p-4 bg-danger-50 border border-danger-200 rounded-md" role="alert" data-testid="label-manager-error">
-        <p className="text-sm text-danger-700">{error}</p>
+        <p className="text-sm text-danger-700">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</p>
       </div>
     );
   }

frontend/src/components/mail/PgpSettings.tsx

@@ -93,7 +93,7 @@ export function PgpSettings() {
   if (error) {
     return (
       <div className="p-4 bg-danger-50 border border-danger-200 rounded-md" role="alert" data-testid="pgp-settings-error">
-        <p className="text-sm text-danger-700">{error}</p>
+        <p className="text-sm text-danger-700">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</p>
       </div>
     );
   }

frontend/src/components/mail/RuleEditor.tsx

@@ -153,7 +153,7 @@ export function RuleEditor({ accountId }: { accountId: string }) {
   if (error) {
     return (
       <div className="p-4 bg-danger-50 border border-danger-200 rounded-md" role="alert" data-testid="rule-editor-error">
-        <p className="text-sm text-danger-700">{error}</p>
+        <p className="text-sm text-danger-700">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</p>
       </div>
     );
   }

frontend/src/components/mail/SignatureManager.tsx

@@ -134,7 +134,7 @@ export function SignatureManager() {
   if (error) {
     return (
       <div className="p-4 bg-danger-50 border border-danger-200 rounded-md" role="alert" data-testid="signature-manager-error">
-        <p className="text-sm text-danger-700">{error}</p>
+        <p className="text-sm text-danger-700">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</p>
       </div>
     );
   }

frontend/src/components/mail/TemplatePicker.tsx

@@ -66,7 +66,7 @@ export function TemplatePicker({ onSelect }: TemplatePickerProps) {
   if (error) {
     return (
       <div className="p-4 bg-danger-50 border border-danger-200 rounded-md" role="alert" data-testid="template-picker-error">
-        <p className="text-sm text-danger-700">{error}</p>
+        <p className="text-sm text-danger-700">{typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}</p>
       </div>
     );
   }