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/package.json

@@ -14,6 +14,7 @@
   "dependencies": {
     "@pubscript/core": "workspace:*",
     "@pubscript/editor": "workspace:*",
+    "@pubscript/exporters": "workspace:*",
     "@pubscript/ui": "workspace:*",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",