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.

electron-builder.config.js

@@ -0,0 +1,124 @@
+/**
+ * electron-builder configuration (replaces the static package.json "build"
+ * section so it can be computed dynamically).
+ *
+ * Why a JS config: the bundled markitdown binary is OPTIONAL at build time —
+ * PyInstaller can only build for the host OS, so e.g. a Windows package
+ * built on a machine without bin/win32/markitdown.exe must simply omit it
+ * (the app falls back to system-installed markitdown at runtime) instead of
+ * failing on a missing extraFiles source.
+ *
+ * Everything else mirrors the previous static config verbatim.
+ */
+const fs = require('fs');
+const path = require('path');
+
+/** Common extraFiles for a platform: pandoc always, markitdown when built. */
+function toolExtraFiles(platform) {
+  const exe = platform === 'win32' ? '.exe' : '';
+  const files = [{ from: `bin/${platform}/pandoc${exe}`, to: `bin/pandoc${exe}` }];
+  const markitdown = `bin/${platform}/markitdown${exe}`;
+  if (fs.existsSync(path.join(__dirname, markitdown))) {
+    files.push({ from: markitdown, to: `bin/markitdown${exe}` });
+  } else {
+    console.warn(
+      `[electron-builder.config] ${markitdown} not found — building WITHOUT the ` +
+        'bundled markitdown (the app will use a system install if present). ' +
+        'Run "npm run bundle:markitdown" on this platform to bundle it.'
+    );
+  }
+  return files;
+}
+
+module.exports = {
+  appId: 'com.concreteinfo.markdownconverter',
+  productName: 'MarkdownConverter',
+  copyright: 'Copyright (C) 2024-2025 ConcreteInfo',
+  directories: { output: 'dist' },
+  icon: 'assets/icon',
+  files: [
+    'src/**/*',
+    'assets/**/*',
+    'scripts/**/*',
+    'node_modules/**/*',
+    'package.json',
+    'THIRD-PARTY-NOTICES.md',
+    'SOURCES.md',
+    'third-party-licenses/**/*',
+  ],
+  asarUnpack: [
+    'node_modules/ffmpeg-static/**',
+    'node_modules/sharp/**',
+    'node_modules/@img/**',
+    'node_modules/@napi-rs/**',
+    'assets/fonts/**',
+  ],
+  fileAssociations: [
+    {
+      ext: 'md',
+      name: 'Markdown Document',
+      description: 'Markdown Document',
+      mimeType: 'text/markdown',
+      role: 'Editor',
+    },
+    {
+      ext: 'markdown',
+      name: 'Markdown Document',
+      description: 'Markdown Document',
+      mimeType: 'text/markdown',
+      role: 'Editor',
+    },
+    {
+      ext: 'pdf',
+      name: 'PDF Document',
+      description: 'PDF Document',
+      mimeType: 'application/pdf',
+      role: 'Editor',
+    },
+  ],
+  mac: {
+    category: 'public.app-category.productivity',
+    identity: null,
+    extraFiles: toolExtraFiles('darwin'),
+  },
+  win: {
+    target: [
+      { target: 'nsis', arch: ['x64'] },
+      { target: 'portable', arch: ['x64'] },
+      { target: 'zip', arch: ['x64'] },
+    ],
+    artifactName: '${productName}-${version}-${arch}.${ext}',
+    requestedExecutionLevel: 'asInvoker',
+    legalTrademarks: 'Copyright (C) 2024-2025 ConcreteInfo',
+    verifyUpdateCodeSignature: false,
+    signAndEditExecutable: false,
+    extraFiles: toolExtraFiles('win32'),
+  },
+  nsis: {
+    oneClick: false,
+    perMachine: false,
+    allowToChangeInstallationDirectory: true,
+    displayLanguageSelector: true,
+    createDesktopShortcut: true,
+    createStartMenuShortcut: true,
+    shortcutName: 'MarkdownConverter',
+    runAfterFinish: true,
+    menuCategory: 'Productivity',
+    license: 'LICENSE',
+    warningsAsErrors: false,
+    artifactName: '${productName}-Setup-${version}.${ext}',
+    deleteAppDataOnUninstall: false,
+    differentialPackage: true,
+  },
+  linux: {
+    target: ['deb', 'AppImage', 'snap'],
+    category: 'Utility',
+    maintainer: 'ConcreteInfo <amit.wh@gmail.com>',
+    extraFiles: toolExtraFiles('linux'),
+  },
+  deb: {
+    depends: ['imagemagick', 'libreoffice-common'],
+    description: 'Professional Markdown editor and universal file converter',
+    maintainer: 'ConcreteInfo <amit.wh@gmail.com>',
+  },
+};