Add browser tests for the Material interface

Upload drop zone, copying a share link, unlocking a download on a phone,
sorting and deleting on the admin dashboard, and the system theme with
the Appearance picker.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V9NnLxnPp8vaaurb3Z1MFy

phpunit.xml

@@ -11,6 +11,9 @@
         <testsuite name="Feature">
             <directory>tests/Feature</directory>
         </testsuite>
+        <testsuite name="Browser">
+            <directory>tests/Browser</directory>
+        </testsuite>
     </testsuites>
     <source>
         <include>