Reword customer file list heading

Files in the customer dir can come from uploads or be placed there by
staff via WebDAV — say so explicitly in the heading.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/upload.html

@@ -177,7 +177,7 @@
     <div id="list"></div>
 
     <div class="browser">
-      <h2>Bisher hochgeladene Dateien <span class="count" id="fileCount"></span></h2>
+      <h2>Bisher hochgeladene und/oder empfangene Dateien <span class="count" id="fileCount"></span></h2>
       <div id="fileBrowser"><div class="empty">– wird geladen –</div></div>
     </div>
   </div>