i18n: inline English fallback for noConverter import message

The original abiword message existed in ~70 locale files and was
removed from all of them by this PR. The replacement key was only
added to en.json, so non-English users had an empty div until
translators localize. Follow the project's usual pad.html pattern
(e.g. line 146's "Font type:") and include the English text inside
the div as the fallback content; html10n replaces it when a
translation is available.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/templates/pad.html

@@ -192,7 +192,7 @@
           <div class="acl-write">
               <% e.begin_block("importColumn"); %>
               <h2 data-l10n-id="pad.importExport.import"></h2>
-              <div class="importmessage" id="importmessagenoconverter" data-l10n-id="pad.importExport.noConverter.innerHTML" hidden></div>
+              <div class="importmessage" id="importmessagenoconverter" data-l10n-id="pad.importExport.noConverter.innerHTML" hidden>You can only import from plain text or HTML formats. For more advanced import features, please <a href="https://github.com/ether/etherpad-lite/wiki/How-to-enable-importing-and-exporting-different-file-formats-with-AbiWord">install LibreOffice</a>.</div>
               <form id="importform" method="post" action="" target="importiframe" enctype="multipart/form-data">
                   <div class="importformdiv" id="importformfilediv">
                       <input type="file" name="file" size="10" id="importfileinput">