feat(processed-audio-sources): phase 8 - frontend design consistency review

Fix audio source modal error class (modal-error), use Modal.showError(),
reorder audio source card description, remove redundant APT filter count
badge, clean up unused imports in audio-sources.ts.

server/src/wled_controller/templates/modals/audio-source-editor.html

@@ -9,7 +9,7 @@
             <form id="audio-source-form" onsubmit="return false;">
                 <input type="hidden" id="audio-source-id">
 
-                <div id="audio-source-error" class="error-message" style="display: none;"></div>
+                <div id="audio-source-error" class="modal-error" style="display: none;"></div>
 
                 <!-- Name -->
                 <div class="form-group">