feat(storyboard): 整张生成时每张分镜转圈(不全屏遮挡)+ 未生成占位回 2:3 竖屏

- 每张分镜占位(缩略图 + 大图)叠半透明遮罩 + spinner,样式同基础资产趴;不再全屏遮挡
- 中间大图空占位由 16:9 横框改 2:3 竖屏,与成品(1024×1536)同形;fh-flat 下加兜底高度居中
- 生成结束(成功/失败)spinner 自动停

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/frontend/src/pipeline-page.css

@@ -385,6 +385,8 @@
   .content--fh-flat .stage[data-stage-pane="3"].active > .stage-storyboard > .sb-canvas > .sb-main-img { width: 100%; }
   /* 有真图时大图占满画布整个高度(不再 16/9 居中留上下空白) */
   .content--fh-flat .stage[data-stage-pane="3"].active > .stage-storyboard > .sb-canvas > .sb-main-img.has-mock-media { align-self: stretch; aspect-ratio: auto; }
+  /* 2:3 空占位:别被上面的 width:100% 撑成超高满框 —— 给个合理高度、按 2:3 推宽、居中 */
+  .content--fh-flat .stage[data-stage-pane="3"].active > .stage-storyboard > .sb-canvas > .sb-main-img:not(.has-mock-media) { width: auto; height: min(70vh, 540px); aspect-ratio: 2/3; justify-self: center; }
   .content--fh-flat .stage[data-stage-pane="3"].active > .stage-storyboard > .sb-side { display: flex; flex-direction: column; min-height: 0; }
   .content--fh-flat .stage[data-stage-pane="3"].active > .stage-storyboard > .sb-side > .pane { flex: 1 1 0; min-height: 0; overflow-y: auto; border: 0; border-radius: 0; background: var(--surface); padding: 18px 28px; }
   .content--fh-flat .stage[data-stage-pane="3"].active .sb-scenes-col { max-height: none; }
@@ -400,10 +402,15 @@
   .sb-scene-thumb .placeholder { aspect-ratio: 1; }
   .sb-scene-thumb .nm { font-size: 11.5px; font-weight: 500; color: var(--accent-black); }
   .sb-scene-thumb .sub { font-family: var(--font-mono); font-size: 10.5px; color: var(--black-alpha-48); }
-  .sb-main-img { aspect-ratio: 16/9; min-height: 0; }
+  /* 整张故事板出图是竖屏 2:3(1024×1536),空占位也用 2:3,跟成品同形(不再 16:9 横框) */
+  .sb-main-img { aspect-ratio: 2/3; min-height: 0; }
   /* 整张故事板是竖长拼图,cover 会裁掉上下 —— 用 contain 完整显示;
      留白底色用白(--surface)而非占位浅灰,避免「灰底大框」(对齐 HTML 干净白底) */
   .sb-main-img.has-mock-media { background-size: contain; background-color: var(--surface); }
+  /* 整张生成中:每张分镜占位上叠一层半透明遮罩 + 转圈 spinner(不全屏遮挡;样式同基础资产趴) */
+  .sb-gen-veil { position: absolute; inset: 0; z-index: 3; display: grid; place-items: center; border-radius: inherit; pointer-events: none; background: color-mix(in srgb, var(--surface) 62%, transparent); }
+  .sb-scene-thumb .sb-gen-veil .spinner { width: 16px; height: 16px; }
+  .sb-main-img .sb-gen-veil .spinner { width: 28px; height: 28px; }
 
   .sb-rerun-note { display: flex; align-items: flex-start; gap: 10px; padding: 10px 12px; margin-bottom: 14px; background: rgba(180,83,9,.08); border: 1px solid rgba(180,83,9,.20); border-radius: var(--r-md); color: #7C3A05; line-height: 1.55; }
   .sb-rerun-note .warn-ic { width: 22px; height: 22px; border-radius: var(--r-sm); background: rgba(180,83,9,.12); color: #B45309; display: grid; place-items: center; flex: 0 0 22px; }