Sub-step 이전/다음 navigation + EULA popup flow

- Each 3-x and 4-x sub-step now has its own [이전][다음] action row at the bottom; 이전 returns to the previous sub-step (or previous main step on the first one) so users can move freely both directions
- 3-3 EULA: replace the inline checkbox with a modal popup. After server zip downloads, the renderer reads eula.txt via server:readEula; if absent, falls back to the live minecraft.net/en-us/eula HTML via server:fetchMinecraftEula and shows it in a sandboxed iframe
- Popup buttons: 동의 → server:acceptEula and proceed to RAM check; 비동의 / X / overlay click → "EULA 동의 실패. 다운로드를 취소합니다." and re-enable 다운로드 시작 for retry
- main.ts: stop auto-deleting eula.txt after server zip extraction so the popup can read whatever the zip provided
- 4-2 install completion now keeps a state.client.clientInstalled flag so backing into 4-2 doesn't force a re-install

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

src/installer/main.ts

@@ -295,11 +295,30 @@ ipcMain.handle('server:install', async (_event, payload: ServerInstallPayload) =
   sendLog(`서버 설치 경로: ${installPath}`)
 
   await downloadServerZip(pack.pack, installPath)
+  // 다운로드한 zip에 들어있을 수 있는 eula.txt를 그대로 보존한다.
+  // 동의 흐름은 renderer가 별도 IPC로 읽고 동의 시 덮어쓴다.
+})
 
-  const eulaPath = path.join(installPath, 'eula.txt')
-  if (fs.existsSync(eulaPath)) {
-    await fsp.unlink(eulaPath)
-    sendLog('기존 eula.txt 삭제, 사용자 동의를 다시 받습니다.')
+ipcMain.handle('server:readEula', async (_event, installPath: string): Promise<{ exists: boolean; content: string }> => {
+  if (!installPath) return { exists: false, content: '' }
+  const target = path.join(path.resolve(installPath), 'eula.txt')
+  try {
+    const content = await fsp.readFile(target, 'utf8')
+    return { exists: true, content }
+  } catch (error) {
+    if ((error as NodeJS.ErrnoException).code === 'ENOENT') return { exists: false, content: '' }
+    throw error
+  }
+})
+
+ipcMain.handle('server:fetchMinecraftEula', async (): Promise<{ url: string; html: string }> => {
+  const url = 'https://www.minecraft.net/en-us/eula'
+  try {
+    const buffer = await fetchBuffer(url)
+    return { url, html: buffer.toString('utf8') }
+  } catch (error) {
+    sendLog(`Minecraft EULA 페이지 조회 실패: ${(error as Error).message}`)
+    return { url, html: '' }
   }
 })