feat(legal): bundle small dependencies, add notices, credits, and GPL source offers

Bundle (v4.7.0):
- MarkItDown frozen to a single ~75MB per-platform binary (PyInstaller via
  npm run bundle:markitdown; ML extras excluded) — built and verified
  locally: HTML/XLSX/PDF conversions pass through the bundled binary, and
  the app resolves bin/linux/markitdown first at runtime
- Packaging copies bundled markitdown alongside Pandoc for win/mac/linux;
  FFmpeg/sharp/KaTeX/fonts were already bundled

Legal artifacts:
- THIRD-PARTY-NOTICES.md: complete license inventory of everything
  distributed (binaries, npm runtime deps, fonts, embedded Python packages)
- SOURCES.md: GPL §3(b) written source offers for Pandoc 3.9.0.2,
  ffmpeg-static's GPL build, and the PyInstaller bootloader; LGPL relinking
  note for libvips
- third-party-licenses/: canonical GPL-2.0, LGPL-2.1, MPL-2.0, Apache-2.0,
  OFL-1.1, PSF-Python texts
- Help > Third-Party Notices & Licenses: in-app viewer for both documents
- README: 'Bundled Dependencies, Legal Notices & Credits' section

Hardening:
- download-tools.js now SHA-256 pins every artifact, verifies after
  download AND against the cache on every run, and hard-fails on mismatch
  (closes security finding D6)

Large tools intentionally not bundled (documented): LibreOffice,
MiKTeX/TeX Live, ImageMagick, PlantUML+JRE, Calibre.

637/637 tests green; lint clean; clean boot; bundled binary verified.

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "markdown-converter",
-  "version": "4.6.1",
+  "version": "4.7.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "markdown-converter",
-      "version": "4.6.1",
+      "version": "4.7.0",
       "license": "MIT",
       "dependencies": {
         "@cantoo/pdf-lib": "^2.9.1",