feat(core/frontend): pipeline stage editor (burn-in controls) + double-submit guard & button greying

Pipeline (脚本→资产→故事板→视频→拼接):
- Stage1 render real script shots + wire 确认脚本→adopt (advance stage)
- Stage2 add person/scene AI-生成 buttons + clickable category tabs
- Stage4 auto-poll videos to completion + per-segment upload + real frame thumbnails + download
- Stage5 real timeline editor: clips undo/redo/split/copy/delete/drag-reorder/zoom,
  subtitle style + per-clip text editor, transition select (xfade preview),
  BGM upload + volume, save draft, export-with-save → shows/download final MP4
- embedded asset URLs everywhere (beat assets pagination)

UX: re-entry guard in action() (no double-submit anywhere) + greyed :disabled
styles for btn-aigen/chat-mode/pill-cta/tl-action so generate buttons visibly
disable while generating.

Also includes prior uncommitted frontend work: settings preferences/sessions/avatar,
asset delete, account/team/products pages, fonts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/frontend/src/library-page.css

@@ -12,4 +12,8 @@
   .asset-name { font-size: 13px; font-weight: 600; color: var(--accent-black); }
   .asset-meta { font-size: 11px; color: var(--black-alpha-48); margin-top: 3px; font-family: var(--font-mono); letter-spacing: .02em; }
   .asset-badge { position: absolute; top: 8px; left: 8px; font-family: var(--font-mono); font-size: 10px; letter-spacing: .04em; padding: 2px 6px; background: var(--surface); border: 1px solid var(--border-faint); border-radius: var(--r-sm); color: var(--black-alpha-56); }
+  .asset-card { position: relative; }
 }
+
+/* 编辑模式:开启「管理资产」后,资产卡删除按钮常显(否则全局只 hover 显) */
+body.edit-mode .asset-card .card-del-btn { opacity: 1 !important; pointer-events: auto !important; }