feat(layout): expose canWrite flag from layout server load

Derives canWrite from WRITE_ALL permission in user groups, available
as page.data.canWrite on every page without per-page boilerplate.

Refs #17
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/+layout.server.ts

@@ -2,6 +2,7 @@ import type { LayoutServerLoad } from './$types';
 
 export const load: LayoutServerLoad = async ({ locals }) => {
     return {
-        user: locals.user
+        user: locals.user,
+        canWrite: locals.user?.groups?.some((g: { permissions: string[] }) => g.permissions.includes('WRITE_ALL')) ?? false
     };
 };