feat(installer): record EULA acceptance timestamp in eula.txt

Write an ISO 8601 "# EULA accepted at: <time>" comment when the user
accepts the EULA, so the agreement time is captured alongside eula=true.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/installer/main.ts

@@ -656,7 +656,12 @@ ipcMain.handle('server:fetchMinecraftEula', async (): Promise<{ url: string; htm
 
 ipcMain.handle('server:acceptEula', async (_event, installPath: string) => {
   const target = path.join(installPath, 'eula.txt')
-  await fsp.writeFile(target, `# Generated by music quiz installer\neula=true\n`, 'utf8')
+  const acceptedAt = new Date()
+  await fsp.writeFile(
+    target,
+    `# Generated by music quiz installer\n# EULA accepted at: ${acceptedAt.toISOString()}\neula=true\n`,
+    'utf8',
+  )
   sendLog(t('log.eulaAccepted'))
 })