feat(export): Chunk #6 Export Pipeline — 7 exporters, ExportDialog & tests

Add the @pubscript/exporters package exporting a flattened ProseMirror
project to Markdown, HTML, EPUB, DOCX, ODT and PDF, plus a deterministic
generatedAt timestamp threaded through every format (ZIP entry dates,
PDF /ID, DOCX dc:created/modified, EPUB id, HTML/XML footers) so golden
files are byte-stable.

Wire an ExportDialog into the web app (Topbar trigger, App route) with
format selection, theme pick, and download helpers.

Add round-trip and golden-file determinism tests (22) covering
non-empty output, byte-identical re-exports, Markdown round-trip through
the editor, and EPUB/ODT ZIP + mimetype + XML validation.

apps/web/src/main.tsx

@@ -8,6 +8,7 @@ import "./styles/entities.css";
 import "./styles/plot.css";
 import "./styles/timeline.css";
 import "./styles/whiteboard.css";
+import "./styles/export.css";
 
 createRoot(document.getElementById("root")!).render(
   <StrictMode>