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/preload.ts

@@ -19,6 +19,10 @@ const api = {
   // 3-3
   startServerInstall: (payload: ServerInstallPayload): Promise<void> =>
     ipcRenderer.invoke('server:install', payload),
+  readEula: (installPath: string): Promise<{ exists: boolean; content: string }> =>
+    ipcRenderer.invoke('server:readEula', installPath),
+  fetchMinecraftEula: (): Promise<{ url: string; html: string }> =>
+    ipcRenderer.invoke('server:fetchMinecraftEula'),
   acceptEula: (installPath: string): Promise<void> =>
     ipcRenderer.invoke('server:acceptEula', installPath),
   checkRam: (packKey: string): Promise<RamCheckResult> =>