feat(frontend): add (app) route group and rework auth routing

Move authenticated pages under an (app) group with a Sidebar shell, add /auth/login and page-based /login, drop the old explore page and login +server endpoint, and add the server upload helper.

frontend/src/routes/auth/login/+server.ts

@@ -0,0 +1,49 @@
+import { redirect } from '@sveltejs/kit';
+import * as client from 'openid-client';
+import type { RequestHandler } from './$types';
+import {
+	getOidcConfig,
+	STATE_COOKIE,
+	VERIFIER_COOKIE,
+	NONCE_COOKIE,
+	RETURN_COOKIE
+} from '$lib/server/auth';
+import { authConfig } from '$lib/server/config';
+
+export const GET: RequestHandler = async ({ url, cookies }) => {
+	const config = await getOidcConfig();
+
+	const codeVerifier = client.randomPKCECodeVerifier();
+	const codeChallenge = await client.calculatePKCECodeChallenge(codeVerifier);
+	const state = client.randomState();
+	const nonce = client.randomNonce();
+
+	const register = url.searchParams.get('mode') === 'register';
+	const returnTo = url.searchParams.get('returnTo') ?? '/';
+
+	const parameters: Record<string, string> = {
+		redirect_uri: authConfig.redirectUri,
+		scope: authConfig.scope,
+		code_challenge: codeChallenge,
+		code_challenge_method: 'S256',
+		state,
+		nonce
+	};
+	if (register) parameters.prompt = 'create';
+
+	const authorizationUrl = client.buildAuthorizationUrl(config, parameters);
+
+	const cookieOptions = {
+		httpOnly: true,
+		sameSite: 'lax' as const,
+		secure: url.protocol === 'https:',
+		path: '/',
+		maxAge: authConfig.flowTtlSeconds
+	};
+	cookies.set(VERIFIER_COOKIE, codeVerifier, cookieOptions);
+	cookies.set(STATE_COOKIE, state, cookieOptions);
+	cookies.set(NONCE_COOKIE, nonce, cookieOptions);
+	cookies.set(RETURN_COOKIE, returnTo, cookieOptions);
+
+	redirect(302, authorizationUrl.href);
+};