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/package.json

@@ -0,0 +1,31 @@
+{
+  "name": "@pubscript/exporters",
+  "version": "0.1.0",
+  "type": "module",
+  "description": "Format-agnostic export engine for PubScript projects (Markdown, HTML, EPUB, DOCX, ODT, PDF).",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "exports": {
+    ".": "./src/index.ts",
+    "./package.json": "./package.json"
+  },
+  "scripts": {
+    "build": "tsc --noEmit",
+    "test": "vitest run",
+    "lint": "eslint .",
+    "typecheck": "tsc --noEmit"
+  },
+  "dependencies": {
+    "@pubscript/core": "workspace:*",
+    "docx": "^8.0.0",
+    "jszip": "^3.10.1",
+    "jspdf": "^2.5.2"
+  },
+  "devDependencies": {
+    "@pubscript/editor": "workspace:*",
+    "@types/node": "^22.10.2",
+    "jsdom": "^25.0.1",
+    "typescript": "^5.7.2",
+    "vitest": "^2.1.8"
+  }
+}