fix(webui): derive the account avatar initial by code point

(displayName).charAt(0) returns the first UTF-16 unit, so a non-BMP first character
(emoji, rare CJK) rendered as a lone surrogate (�). Spread to iterate by code point.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/pages/settings/settings-account.test.tsx

@@ -32,6 +32,18 @@ describe('SettingsAccount gating', () => {
         expect(screen.getAllByRole('button', { name: 'Change' })).toHaveLength(3);
     });
 
+    it.each([
+        ['😀 Team', '😀'],
+        ['中文 用户', '中'],
+        ['한국 사용자', '한'],
+        ['𠀋 Lin', '𠀋'],
+    ])('uses the first whole code point of %s as the avatar initial', (name, expected) => {
+        authState.value = { user: { mail: 'e@x.com', name, type: 'local' } };
+        render(<SettingsAccount />);
+
+        expect(screen.getByText(expected)).toBeInTheDocument();
+    });
+
     it('hides password and email editing for an OAuth account but keeps the name editable', () => {
         authState.value = { user: githubUser };
         render(<SettingsAccount />);

frontend/src/pages/settings/settings-account.tsx

@@ -31,7 +31,7 @@ function SettingsAccount() {
     const stopEditing = () => setEditing(null);
 
     const displayName = user.name?.trim() || user.mail;
-    const initial = (displayName || '?').charAt(0).toUpperCase();
+    const initial = ([...(displayName || '?')][0] ?? '?').toUpperCase();
     const createdAt = user.created_at ? new Date(user.created_at) : null;
     const memberSince =
         createdAt && !Number.isNaN(createdAt.getTime()) ? format(createdAt, 'MMMM yyyy', { locale: enUS }) : null;