Set default value for markdown preview and improve error for collab

web-app/src/routes/(authenticated)/account/+page.svelte

@@ -66,3 +66,9 @@
     </form>
   </Modal>
 </section>
+
+<style>
+  form[action="?/logout"] > button {
+    max-inline-size: fit-content;
+  }
+</style>

web-app/src/routes/(authenticated)/website/[websiteId]/collaborators/+page.server.ts

@@ -41,6 +41,10 @@ export const actions: Actions = {
       )
     ).data;
 
+    if (!user) {
+      return { success: false, message: "Specified user could not be found" };
+    }
+
     return await apiRequest(fetch, `${API_BASE_PREFIX}/collab`, "POST", {
       body: {
         website_id: params.websiteId,