fix: align form field names in skin/cape upload (file → skin/cape)

JS sent fd.append('file', ...) but Go expected r.FormFile("skin") / r.FormFile("cape").

internal/templates/html/profile.html

@@ -140,7 +140,7 @@
         const file = document.getElementById('skinFile').files[0];
         if (!file) return;
         const fd = new FormData();
-        fd.append('file', file);
+        fd.append('skin', file);
         const alert = document.getElementById('alertSkin');
         try {
             const res = await fetch('/api/web/profile/skin', {
@@ -169,7 +169,7 @@
         const file = document.getElementById('capeFile').files[0];
         if (!file) return;
         const fd = new FormData();
-        fd.append('file', file);
+        fd.append('cape', file);
         const alert = document.getElementById('alertCape');
         try {
             const res = await fetch('/api/web/profile/cape', {