fix: revalidate all pages on admin save so hero/config changes appear immediately

Add revalidatePath('/', 'layout') calls to gallery save, settings save,
and reload routes. This tells Next.js to purge any cached page responses
so the next browser refresh picks up the new hero images and config.

app/api/admin/settings/route.ts

@@ -1,4 +1,5 @@
 import { NextResponse } from 'next/server';
+import { revalidatePath } from 'next/cache';
 import { isAdminAuthenticated, isAdminEnabled } from '@/lib/admin/auth';
 import { readSettingsYaml, writeSettingsYaml } from '@/lib/admin/yaml-service';
 import { invalidateConfigCache } from '@/lib/config';
@@ -38,6 +39,7 @@ export async function PUT(request: Request) {
     await writeSettingsYaml(settings);
     invalidateConfigCache();
     immich.invalidateAll();
+    revalidatePath('/', 'layout');
     return NextResponse.json({ success: true, message: 'Saved successfully. Backup of previous version created.' });
   } catch (err) {
     console.error('[Admin] Failed to write settings.yaml:', err);