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/routes/api/progress/+server.ts

@@ -1,6 +1,7 @@
 import { json, error } from '@sveltejs/kit';
 import type { RequestHandler } from './$types';
 import { setProgress } from '$lib/server/pocketbase';
+import { log } from '$lib/server/logger';
 
 /**
  * POST /api/progress
@@ -14,6 +15,11 @@ export const POST: RequestHandler = async ({ request, locals }) => {
 		error(400, 'Invalid body — expected { slug, chapter }');
 	}
 
-	await setProgress(locals.sessionId, body.slug, body.chapter);
+	try {
+		await setProgress(locals.sessionId, body.slug, body.chapter);
+	} catch (e) {
+		log.error('progress', 'setProgress failed', { slug: body.slug, chapter: body.chapter, err: String(e) });
+		error(500, 'Failed to save progress');
+	}
 	return json({ ok: true });
 };