feat: editable branding — app name and subtitle configurable in Admin Settings

Adds app_name and app_subtitle as system settings with a dedicated
Branding tab in the Admin panel. Both values are served via a public
GET /api/admin/branding endpoint (no auth) so the login page can also
show the configured name before the user signs in.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/branding.ts

@@ -0,0 +1,11 @@
+import api from './client'
+
+export interface BrandingSettings {
+  app_name: string
+  app_subtitle: string
+}
+
+export async function fetchBranding(): Promise<BrandingSettings> {
+  const { data } = await api.get<BrandingSettings>('/admin/branding')
+  return data
+}