fix(browse): bundle shared/zddc-source.js so downloadConverted is available

The markdown editor's DOCX/HTML/PDF download buttons silently no-op'd
because the gate `typeof window.zddc.source.downloadConverted ===
'function'` always failed: browse rolls its own server-mode detection
(state.source === 'server' + node.url) and never needed the shared/
zddc-source.js polyfill before. The new download helper lives on
window.zddc.source, so browse needs to bundle it.

Adds ../shared/zddc-source.js to browse/build.sh's concat_files list,
right after preview-lib.js. Bundle gains ~12 KB; tools that don't
need the polyfill (browse doesn't use HttpDirectoryHandle directly)
pay a small footprint cost in exchange for getting the helper.

browse/build.sh

@@ -49,6 +49,7 @@ concat_files \
     "../shared/logo.js" \
     "../shared/help.js" \
     "../shared/preview-lib.js" \
+    "../shared/zddc-source.js" \
     "js/init.js" \
     "js/loader.js" \
     "js/tree.js" \