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.

tests/packaging-sharp.test.js

@@ -50,8 +50,11 @@ function listFilesRecursively(dir) {
   );
 
   test('sharp-linux-x64 ships its native binding', () => {
-    const binding = path.join(UNPACKED_IMG_DIR, 'sharp-linux-x64', 'lib', 'sharp-linux-x64.node');
-    expect(fs.existsSync(binding)).toBe(true);
+    // sharp 0.35 names the binding with a version suffix
+    // (sharp-linux-x64-0.35.4.node); 0.34 used a bare name — accept both
+    const libDir = path.join(UNPACKED_IMG_DIR, 'sharp-linux-x64', 'lib');
+    const bindings = fs.readdirSync(libDir).filter((f) => /^sharp-linux-x64.*\.node$/.test(f));
+    expect(bindings.length).toBeGreaterThan(0);
   });
 
   test('sharp-libvips-linux-x64 ships the bundled libvips shared libraries', () => {