fix(planner): map backend role 'planner' to 'planer' and enlarge nav buttons to 40px touch targets

- hooks.server.ts: replace type-cast with actual mapping so isPlanner works
- planner page: set min-h/min-w 40px on prev/next/heute week buttons

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

frontend/src/hooks.server.ts

@@ -39,7 +39,7 @@ export const handle: Handle = async ({ event, resolve }) => {
 	event.locals.benutzer = {
 		id: user.id!,
 		name: user.displayName!,
-		rolle: (user.householdRole as 'planer' | 'mitglied') ?? 'mitglied'
+		rolle: user.householdRole === 'planner' ? 'planer' : 'mitglied'
 	};
 	event.locals.haushalt = {
 		id: user.householdId ?? undefined,