Allow saving elements without a URL (#33)

The URL field is optional, but canSave required a valid URL, so an
element with no URL could never be saved. Permit an empty URL while
still rejecting a non-empty invalid one.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dfalling

src/map/ElementEditScreen.tsx

@@ -82,7 +82,11 @@ function EditForm({element, onDone}: {element: Element; onDone: () => void}) {
   const uriError =
     trimmedUri.length > 0 && !uriValid ? 'Enter a valid URL.' : null;
 
-  const canSave = name.trim().length > 0 && uriValid && !saving && !uploading;
+  const canSave =
+    name.trim().length > 0 &&
+    (trimmedUri.length === 0 || uriValid) &&
+    !saving &&
+    !uploading;
 
   async function onAddPhoto() {
     setErrorMessage(null);