feat: add case-insensitive webp support for lora cover photos

Make preview file discovery case-insensitive so files with uppercase
extensions like .WEBP are found on case-sensitive filesystems. Also
explicitly list image/webp in the file picker accept attribute for
broader browser compatibility.

https://claude.ai/code/session_01SgT2pkisi27bEQELX5EeXZ

static/js/api/baseModelApi.js

@@ -251,7 +251,7 @@ export class BaseModelApiClient {
     replaceModelPreview(filePath) {
         const input = document.createElement('input');
         input.type = 'file';
-        input.accept = 'image/*,video/mp4';
+        input.accept = 'image/*,image/webp,video/mp4';
 
         input.onchange = async () => {
             if (!input.files || !input.files[0]) return;