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.

packages/exporters/tsconfig.json

@@ -0,0 +1,9 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "jsx": "react-jsx",
+    "outDir": "./dist",
+    "rootDir": "./src"
+  },
+  "include": ["src/**/*.ts"]
+}