fix(mail): account creation form - add username and is_shared fields

- Add username field to account creation form (defaults to email if empty)
- Add is_shared checkbox toggle for shared mailboxes
- Update CreateAccountPayload type with optional username and is_shared
- The infinite re-render fix (previous commit) resolves the main issue:
  accounts were not shown in settings because loadAccounts was looping,
  flooding the browser with requests and preventing API calls from completing

frontend/src/pages/MailSettings.tsx

@@ -40,7 +40,9 @@ export function MailSettingsPage() {
     imap_port: 993,
     smtp_host: '',
     smtp_port: 587,
+    username: '',
     password: '',
+    is_shared: false,
   });
   const [saving, setSaving] = useState(false);
   const [testing, setTesting] = useState<string | null>(null);
@@ -78,7 +80,9 @@ export function MailSettingsPage() {
         imap_port: 993,
         smtp_host: '',
         smtp_port: 587,
+        username: '',
         password: '',
+        is_shared: false,
       });
     } catch (err: any) {
       toast.error(err?.message || err?.detail || 'Save failed');
@@ -161,6 +165,12 @@ export function MailSettingsPage() {
                   onChange={(e) => setNewAccount((prev) => ({ ...prev, display_name: e.target.value }))}
                   placeholder="John Doe"
                 />
+                <Input
+                  label="Benutzername (IMAP/SMTP)"
+                  value={newAccount.username}
+                  onChange={(e) => setNewAccount((prev) => ({ ...prev, username: e.target.value }))}
+                  placeholder="Leer lassen für E-Mail-Adresse"
+                />
                 <div className="grid grid-cols-2 gap-3">
                   <Input
                     label={t('mail.imapHost')}
@@ -196,6 +206,15 @@ export function MailSettingsPage() {
                   onChange={(e) => setNewAccount((prev) => ({ ...prev, password: e.target.value }))}
                   required
                 />
+                <label className="flex items-center gap-2 text-sm text-secondary-700">
+                  <input
+                    type="checkbox"
+                    checked={newAccount.is_shared || false}
+                    onChange={(e) => setNewAccount((prev) => ({ ...prev, is_shared: e.target.checked }))}
+                    className="rounded"
+                  />
+                  Geteiltes Postfach (für alle Tenant-Benutzer sichtbar)
+                </label>
                 <div className="flex gap-2">
                   <Button onClick={handleCreateAccount} isLoading={saving} size="sm">{t('common.save')}</Button>
                   <Button variant="secondary" size="sm" onClick={() => setShowAddAccount(false)}>{t('common.cancel')}</Button>