Fix small bugs

frontend/src/components/BackLink.tsx

@@ -1,14 +1,27 @@
 import { A } from "@solidjs/router"
+import { Show } from "solid-js"
 
 type BackLinkProps = {
-  href: string
+  // Omit to pop the previous history entry instead of navigating to a fixed
+  // route — for pages reachable from several places, "back" should land the
+  // user wherever they came from.
+  href?: string
   label: string
 }
 
 export default function BackLink(props: BackLinkProps) {
   return (
     <div class="flex items-center gap-2 mb-6">
-      <A href={props.href} class="text-gray-500 hover:text-gray-700">← {props.label}</A>
+      <Show
+        when={props.href}
+        fallback={
+          <button type="button" onClick={() => history.back()} class="text-gray-500 hover:text-gray-700">
+            ← {props.label}
+          </button>
+        }
+      >
+        {(href) => <A href={href()} class="text-gray-500 hover:text-gray-700">← {props.label}</A>}
+      </Show>
     </div>
   )
 }