build(release): dynamic builder config + fix bundled-tool lookup in packages

- electron-builder config moves to electron-builder.config.js so markitdown
  bundling is conditional per platform (PyInstaller only builds for the host
  OS; a missing binary now logs a warning and ships without it instead of
  failing the build); package.json static build section removed, all npm
  scripts pointed at the config; third-party-licenses/ added to packaged files
- FIX (pre-existing): packaged apps looked for bundled pandoc in
  resources/bin, but extraFiles land next to the executable (Contents/ on
  macOS) — packaged builds silently fell back to system pandoc since 4.5.
  New bundledToolDir() resolves the real location for pandoc + markitdown
- download-tools.js pins the win32 pandoc.exe SHA-256 (fetched + verified)
- sharp packaging test accepts sharp 0.35's versioned binding filename
- release.yml: bundle-markitdown step (best-effort) on every OS and a new
  macOS job; release aggregates linux+windows+macos artifacts

Local release artifacts built and verified (dist/):
- MarkdownConverter-4.7.0.AppImage (363MB, pandoc+markitdown bundled,
  packaged app boots clean, tools resolve at the fixed path)
- markdown-converter_4.7.0_amd64.deb (293MB)
- MarkdownConverter-Setup-4.7.0.exe (223MB), portable exe (223MB), zip
  (305MB) — pandoc bundled; markitdown omitted (cannot cross-build),
  legal docs verified inside app.asar

637/637 tests green; lint clean.

scripts/download-tools.js

@@ -27,9 +27,9 @@ const PANDOC_VERSION = '3.9.0.2';
  */
 const KNOWN_SHA256 = {
   'linux:pandoc': '7d124235998ecd3cdd9a463b1e5f6691a178b6461824c29a36170a0882f05597',
+  'win32:pandoc.exe': 'e83f8354c0f507222b5684797b9c5ae766f03889785995d14aac27816ec456ba',
   // Fill these from a trusted machine after the first download of each
   // platform (the script prints the computed hash):
-  // 'win32:pandoc.exe': '…',
   // 'darwin:pandoc': '…',
   'fonts:FiraCode-Regular.ttf': '3c79d234a9161c790410ebb2a80de7efb7c15f581062c130e0fa78503ccdd0da',
   'fonts:FiraCode-Bold.ttf': '975f26779fac1029c2cbdac1e9fac7e9ddeec05e064675e4aac63bffa121742f',