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.

.github/workflows/release.yml

@@ -25,6 +25,13 @@ jobs:
       - name: Download external tools (pandoc)
         run: node scripts/download-tools.js
 
+      - name: Bundle MarkItDown (optional, best effort)
+        # Freezes markitdown + Python into bin/linux/markitdown; failure is
+        # non-fatal — the build then ships without it and the app falls back
+        # to a system markitdown at runtime.
+        run: npm run bundle:markitdown
+        continue-on-error: true
+
       - name: Run tests
         run: npm test
 
@@ -59,6 +66,12 @@ jobs:
       - name: Download external tools (pandoc)
         run: node scripts/download-tools.js
 
+      - name: Bundle MarkItDown (optional, best effort)
+        # Windows runners ship python + venv; non-fatal on failure — the
+        # package then omits the bundled binary by design
+        run: npm run bundle:markitdown
+        continue-on-error: true
+
       - name: Run tests
         run: npm test
 
@@ -94,8 +107,44 @@ jobs:
             dist/*.zip
           retention-days: 5
 
+  build-macos:
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Download external tools (pandoc)
+        run: node scripts/download-tools.js
+
+      - name: Bundle MarkItDown (optional, best effort)
+        run: npm run bundle:markitdown
+        continue-on-error: true
+
+      - name: Run tests
+        run: npm test
+
+      - name: Build macOS packages
+        run: npm run build:mac -- --publish=never
+
+      - name: Upload macOS artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          name: macos-artifacts
+          path: |
+            dist/*.dmg
+            dist/*.zip
+          retention-days: 5
+
   release:
-    needs: [build-linux, build-windows]
+    needs: [build-linux, build-windows, build-macos]
     if: always()
     runs-on: ubuntu-latest
     steps:
@@ -115,6 +164,13 @@ jobs:
           name: windows-artifacts
           path: dist
 
+      - name: Download macOS artifacts
+        uses: actions/download-artifact@v4
+        continue-on-error: true
+        with:
+          name: macos-artifacts
+          path: dist
+
       - name: Create GitHub Release
         uses: softprops/action-gh-release@v2
         with: