bookmarks: don't fabricate a local save when the server rejects it (#121)

frontend/static/bookmarks-ui.js

@@ -111,6 +111,10 @@ function buildStar() {
     try {
       if (existing) await bookmarks.remove(existing.id);
       else await bookmarks.add(loc.lat, loc.lon, currentLabel());
+    } catch (e) {
+      // A real server rejection (e.g. the per-user cap) — bookmarks.add() no
+      // longer fakes a local save for this, so say why nothing changed.
+      alert(e.message || "Couldn't save this location.");
     } finally {
       btn.disabled = false;
     }

frontend/static/bookmarks.js

@@ -97,17 +97,34 @@ async function syncFromServer(preSyncLocal) {
   notify();
 }
 
-/** Save (or re-save with a new label) the given spot. Upserts either way. */
+/** Save (or re-save with a new label) the given spot. Upserts either way.
+ *
+ * Throws if signed in and the server explicitly rejects the request (e.g. the
+ * 409 per-user cap, a 422 on bad input) — that is a real failure, not the
+ * "offline" case below, and must not be papered over with a local-only
+ * bookmark that doesn't actually exist server-side and would just vanish,
+ * unexplained, on the next sync. A genuine network failure (no response at
+ * all) still falls through to the local-only save so the star visibly
+ * sticks even offline. */
 export async function add(lat, lon, label) {
   const id = cellId(lat, lon);
   if (signedIn) {
+    let res = null;
     try {
-      const res = await apiFetch(uv("bookmarks"), { method: "POST", json: { lat, lon, label: label || undefined } });
+      res = await apiFetch(uv("bookmarks"), { method: "POST", json: { lat, lon, label: label || undefined } });
+    } catch (e) {
+      // offline / network failure: fall through to the local-only path below.
+    }
+    if (res) {
       if (res.ok) {
         const rows = await fetchServerList();
         if (rows != null) { cache = rows.map(fromServerRow); writeLocal(cache); notify(); return find(lat, lon); }
+      } else {
+        let msg = `Couldn't save this location (${res.status}).`;
+        try { const d = await res.json(); if (typeof d.detail === "string") msg = d.detail; } catch (e) { /* no body */ }
+        throw new Error(msg);
+      }
     }
-    } catch (e) { /* fall through so the star still visibly sticks, even offline */ }
   }
   let b = cache.find((x) => cellId(x.lat, x.lon) === id);
   if (b) { if (label) b.label = label; }