feat(ui): add structured JSON logging to all server-side routes and lib

Introduces a logger.ts module emitting slog-compatible JSON lines to stderr.
Replaces silent catch blocks and console.error calls throughout minio.ts,
pocketbase.ts, hooks.server.ts, login, books, browse, and all API routes so
auth/registration failures, MinIO presign errors, and scraper proxy failures
are now visible in container logs.

ui/src/hooks.server.ts

@@ -1,6 +1,7 @@
 import type { Handle } from '@sveltejs/kit';
 import { randomBytes, createHmac } from 'node:crypto';
 import { env } from '$env/dynamic/private';
+import { log } from '$lib/server/logger';
 
 const SESSION_COOKIE = 'libnovel_session';
 const AUTH_COOKIE = 'libnovel_auth';
@@ -80,7 +81,15 @@ export const handle: Handle = async ({ event, resolve }) => {
 
 	// Auth cookie → resolve logged-in user
 	const authToken = event.cookies.get(AUTH_COOKIE);
-	event.locals.user = authToken ? parseAuthToken(authToken) : null;
+	if (authToken) {
+		const user = parseAuthToken(authToken);
+		if (!user) {
+			log.warn('auth', 'auth cookie present but failed to parse (malformed or tampered)');
+		}
+		event.locals.user = user;
+	} else {
+		event.locals.user = null;
+	}
 
 	return resolve(event);
 };