fix: React infinite loop - use primitive deps for toolbar registration

- Replace selectedMail object with selectedMailId and selectedMailFlagged primitives
- Prevents useEffect infinite re-render loop

frontend/src/pages/Mail.tsx

@@ -283,8 +283,10 @@ export function MailPage() {
 
   // Register toolbar items
   const { registerItems, unregisterPlugin } = usePluginToolbarStore();
+  const selectedMailId = selectedMail?.id;
+  const selectedMailFlagged = selectedMail?.is_flagged;
   useEffect(() => {
-    const hasMail = !!selectedMail;
+    const hasMail = !!selectedMailId;
     registerItems('mail', [
       {
         id: 'account-select',
@@ -337,7 +339,7 @@ export function MailPage() {
         label: 'Flagge',
         group: 'mail-actions',
         disabled: !hasMail,
-        active: selectedMail?.is_flagged,
+        active: selectedMailFlagged,
         icon: <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M3 21v-4m0 0V5a2 2 0 012-2h6.5l1 1H21l-3 6 3 6h-8.5l-1-1H5a2 2 0 00-2 2zm9-13.5V9" /></svg>,
         onClick: () => selectedMail && handleToggleFlag(selectedMail),
       },
@@ -368,7 +370,7 @@ export function MailPage() {
       },
     ]);
     return () => unregisterPlugin('mail');
-  }, [registerItems, unregisterPlugin, accounts, selectedAccountId, selectedMail, handleAccountSwitch, handleSearch, handleCompose, handleReply, handleForward, handleToggleFlag, handleCreateEvent]);
+  }, [registerItems, unregisterPlugin, accounts, selectedAccountId, selectedMailId, selectedMailFlagged, handleAccountSwitch, handleSearch, handleCompose, handleReply, handleForward, handleToggleFlag, handleCreateEvent]);
 
   if (loadingAccounts) {
     return (