fix(ccusage): resolve ESLint violations in data-loader.ts (#715)

Author: ryoppippi

apps/ccusage/src/data-loader.ts

@@ -18,6 +18,7 @@ import type {
 SortOrder,
 Version,
 } from './_types.ts';
+import { Buffer } from 'node:buffer';
 import { createReadStream, createWriteStream } from 'node:fs';
 import { readFile } from 'node:fs/promises';
 import path from 'node:path';
@@ -4166,7 +4167,7 @@ invalid json line
 
 it('should process large files (600MB+) without RangeError', async () => {
 // Create a realistic JSONL entry similar to actual Claude data (~283 bytes per line)
-const sampleEntry = JSON.stringify({
+const sampleEntry = `${JSON.stringify({
 timestamp: '2025-01-10T10:00:00Z',
 message: {
 id: 'msg_01234567890123456789',
@@ -4175,7 +4176,7 @@ invalid json line
 },
 requestId: 'req_01234567890123456789',
 costUSD: 0.01,
-}) + '\n';
+})}\n`;
 
 // Target 600MB file (this would cause RangeError with readFile in Node.js)
 const targetMB = 600;
@@ -4201,7 +4202,7 @@ invalid json line
 
 // Ensure all data is flushed
 await new Promise<void>((resolve, reject) => {
-writeStream.end((err?: Error | null) => err ? reject(err) : resolve());
+writeStream.end((err?: Error | null) => (err != null) ? reject(err) : resolve());
 });
 
 // Test streaming processing