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>

installer/styles.css

@@ -155,6 +155,67 @@ main {
 
 .toggleRow { display: flex; align-items: center; gap: 10px; margin: 8px 0; }
 
+.modalOverlay {
+  position: fixed;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.6);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  z-index: 1000;
+}
+
+.modalCard {
+  background: var(--bg-alt);
+  border: 1px solid var(--border);
+  border-radius: 12px;
+  width: min(720px, 92vw);
+  max-height: 86vh;
+  display: grid;
+  grid-template-rows: auto 1fr auto;
+  overflow: hidden;
+}
+
+.modalCard > header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 12px 16px;
+  border-bottom: 1px solid var(--border);
+}
+
+.modalCard > header h3 { margin: 0; font-size: 16px; }
+.modalClose { background: transparent; border: none; color: var(--text-muted); font-size: 22px; cursor: pointer; }
+.modalClose:hover { color: var(--text); }
+
+.modalBody {
+  padding: 16px;
+  overflow-y: auto;
+}
+
+.modalCard > footer { padding: 12px 16px; border-top: 1px solid var(--border); margin: 0; }
+
+.eulaPre {
+  background: var(--bg);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  padding: 12px;
+  font-family: 'Consolas', monospace;
+  font-size: 12px;
+  white-space: pre-wrap;
+  word-break: break-word;
+  max-height: 50vh;
+  overflow-y: auto;
+}
+
+.eulaFrame {
+  width: 100%;
+  height: 50vh;
+  background: white;
+  border: 1px solid var(--border);
+  border-radius: 8px;
+}
+
 .statusBadge { display: inline-flex; padding: 3px 10px; border-radius: 999px; font-size: 12px; }
 .statusBadge.pending { background: #2c3849; color: var(--text-muted); }
 .statusBadge.ok { background: rgba(63, 185, 80, 0.2); color: var(--success); }