chore: update @auth/sveltekit to v1.11.1 and remove direct @auth/core dependency

Updated authentication packages to latest versions for security fixes:
- @auth/sveltekit: 1.10.0 → 1.11.1 (includes nodemailer security fix)
- @auth/core: removed from devDependencies (transitively pulled as 0.41.1)

Changed imports to use @auth/sveltekit/providers instead of @auth/core/providers
and removed unused imports from hooks.server.ts.

src/auth.ts

@@ -1,5 +1,5 @@
 import { SvelteKitAuth } from "@auth/sveltekit"
-import Authentik from "@auth/core/providers/authentik"
+import Authentik from "@auth/sveltekit/providers/authentik"
 import { AUTHENTIK_ID, AUTHENTIK_SECRET, AUTHENTIK_ISSUER } from "$env/static/private";
 
 export const { handle, signIn, signOut } = SvelteKitAuth({