feat(IMP-14): A-4 — slide_base embedded vs standalone mode contract

Step 13 owns iframe-vs-standalone CSS contract in slide_base.html via
3-valued embedded_mode enum (auto / embedded / standalone). Removes
SlideCanvas.tsx runtime CSS injection workaround; frontend now passes
?embedded=1 query so auto-mode script attaches html.embedded class and
scopes the standalone body centering/min-height/padding reset.

- templates/phase_z2/slide_base.html: conditional html.embedded class +
  CSP-safe auto-mode <script> + additive html.embedded body/.slide rules
- src/phase_z2_pipeline.py: render_slide gains keyword-only embedded_mode
  ("auto" default) + ValueError guard; 3 existing call sites unchanged
- Front/client/src/components/SlideCanvas.tsx: derive embeddedSrc with
  ?embedded=1 (query-preserving), drop reset CSS injection block
- tests/phase_z2/test_slide_base_embedded_mode.py: 6 cases — auto script,
  CSS rules, embedded/standalone explicit modes, byte-determinism,
  invalid-mode guard

Front/client/src/components/SlideCanvas.tsx

@@ -186,6 +186,12 @@ export default function SlideCanvas({
   // 슬라이드 박스 표시 조건 — final.html 있거나 pendingLayout 모드.
   const showSlideBox = (finalHtmlUrl || isPendingLayout) && !isPipelineRunning;
 
+  // IMP-14 (Step 13 A-4) — backend slide_base.html 가 embedded vs standalone CSS
+  // contract 를 `?embedded=1` query 로 소유. 기존 query string 보존하면서 flag 만 추가.
+  const embeddedSrc = finalHtmlUrl
+    ? `${finalHtmlUrl}${finalHtmlUrl.includes("?") ? "&" : "?"}embedded=1`
+    : undefined;
+
   // wrapper 는 scaled 크기를 가지므로 layout 상 fit. 안의 슬라이드는 1280×720 으로
   // top-left origin scale 후 wrapper 안에 정확히 맞춤.
   const W_SCALED = SLIDE_W * scale;
@@ -283,37 +289,21 @@ export default function SlideCanvas({
           >
             <iframe
               ref={iframeRef}
-              src={finalHtmlUrl}
+              src={embeddedSrc}
               title="Phase Z 렌더 결과"
               className="w-full h-full border-0 block"
               scrolling="no"
               sandbox="allow-same-origin"
               style={{ pointerEvents: isEditMode ? "auto" : "none" }}
               onLoad={(e) => {
-                // final.html 은 standalone 표시용으로 body 에 padding / flex center /
-                // min-height: 100vh 가 있어서, iframe 안에서는 슬라이드가 잘림.
-                // .slide (1280×720) 만 보이도록 reset CSS 를 contentDocument 에 주입.
+                // IMP-14 (Step 13 A-4) — embedded vs standalone CSS reset 은 backend
+                // slide_base.html 가 `?embedded=1` query 로 소유. frontend 가 더 이상
+                // reset CSS 를 contentDocument 에 inject 하지 않음. embedded query 가
+                // backend auto-mode detection script 를 trigger 해서 html.embedded
+                // class 를 붙이고 standalone-only body 규칙을 reset.
                 try {
                   const doc = (e.currentTarget as HTMLIFrameElement).contentDocument;
                   if (!doc) return;
-                  const style = doc.createElement("style");
-                  style.textContent = `
-                    html, body {
-                      margin: 0 !important;
-                      padding: 0 !important;
-                      min-height: 0 !important;
-                      height: 720px !important;
-                      width: 1280px !important;
-                      background: transparent !important;
-                      display: block !important;
-                      overflow: hidden !important;
-                    }
-                    .slide {
-                      box-shadow: none !important;
-                      margin: 0 !important;
-                    }
-                  `;
-                  doc.head.appendChild(style);
 
                   // 2026-05-14 — slide-level override CSS (catalog/template 무변).
                   // Home 이 mdx 별 default visual 보완 (bullet 간격 / zone 비율 등) 지정.