Add better error handling for WebDAV backup response

Parse response as text first, then JSON to capture server errors.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

webapp/settings.php

@@ -1606,7 +1606,15 @@ async function backupToWebDAV() {
             })
         });
 
-        const data = await response.json();
+        const text = await response.text();
+        let data;
+        try {
+            data = JSON.parse(text);
+        } catch (parseError) {
+            console.error("Failed to parse response:", text);
+            throw new Error("Server returned invalid response. Check error logs.");
+        }
+
         if (!data.success) {
             throw new Error(data.error || "WebDAV backup failed");
         }