fix(web): error handling on discard and history toggle

- LabDraft.tsx discard button: chain .catch to surface a failed discard in
  the existing error banner instead of an unhandled rejection.
- LabDraw.tsx toggleHistory: wrap the marker-history fetch in try/catch so a
  failed request silently no-ops (the row just doesn't expand) rather than
  throwing unhandled from the click handler.

web/src/pages/LabDraft.tsx

@@ -80,7 +80,18 @@ export function LabDraft() {
           <button disabled={busy || !collectedDate || markers.length === 0} onClick={confirm}>
             {busy ? "Saving…" : `Confirm ${markers.length} markers`}
           </button>
-          <button className="danger" onClick={() => id && api.discardDraft(id).then(() => nav("/labs"))}>Discard</button>
+          <button
+            className="danger"
+            onClick={() =>
+              id &&
+              api
+                .discardDraft(id)
+                .then(() => nav("/labs"))
+                .catch((e) => setError(e instanceof Error ? e.message : "discard failed"))
+            }
+          >
+            Discard
+          </button>
         </div>
       )}
     </section>