Explain the DeepL glossary limit instead of forwarding the raw error

Saving terms for a domain/direction that has no glossary yet skips the
delete-before-create step, so the create runs against an account whose
glossary allowance may already be spent. DeepL answers 456 with a bare
{"message":"Too many glossaries"}, which was passed straight to the browser
and reads like a bug in this app.

On the API Free plan the allowance is one glossary for the entire account,
so this triggers as soon as a second domain or direction is used — the UI
offers eight slots (4 domains x 2 directions) that the plan cannot back.

The 456 case now reports which glossaries hold the slots and names the two
ways out: free one up, or upgrade the plan.

Co-Authored-By: Claude <noreply@anthropic.com>

server.js

@@ -224,7 +224,18 @@ async function handleSaveGlossary(req, res, domainId, direction) {
       domains[domainId].glossaries[direction] = null;
       saveDomains(domains);
     }
-    return sendJson(res, createResp.status, { error: await forwardDeepLError(createResp) });
+    const detail = await forwardDeepLError(createResp);
+    // DeepL answers 456 "Too many glossaries" once the account's glossary
+    // allowance is used up. On the API Free plan that allowance is a single
+    // glossary for the whole account, so this fires as soon as a second
+    // domain/direction is used - which reads like an app bug unless we say
+    // which glossary is holding the only slot.
+    if (createResp.status === 456 && /too many glossaries/i.test(detail)) {
+      return sendJson(res, 456, {
+        error: await describeGlossaryLimit(base, domainId, direction),
+      });
+    }
+    return sendJson(res, createResp.status, { error: detail });
   }
   const created = await createResp.json();
 
@@ -296,6 +307,33 @@ async function parseJsonBody(req) {
   }
 }
 
+// Turns DeepL's bare "Too many glossaries" into something the person in the
+// browser can act on: which glossaries already occupy the account's slots,
+// and what the two ways out are. Falls back to a generic message if the
+// account can't be listed for some reason.
+async function describeGlossaryLimit(base, domainId, direction) {
+  const wanted = `${DOMAIN_LABELS[domainId]} (${GLOSSARY_DIRECTIONS[direction].label})`;
+  let inUse = [];
+  try {
+    const resp = await fetch(`${base}/v2/glossaries`, {
+      headers: { Authorization: `DeepL-Auth-Key ${DEEPL_API_KEY}` },
+    });
+    if (resp.ok) inUse = (await resp.json()).glossaries || [];
+  } catch (_) {
+    /* fall through to the generic wording below */
+  }
+  const listed = inUse.length
+    ? ` Currently in use: ${inUse.map((g) => `"${g.name}"`).join(', ')}.`
+    : '';
+  return (
+    `Your DeepL plan's glossary limit is already reached, so the glossary for ` +
+    `${wanted} could not be created.${listed} ` +
+    `Delete a glossary you no longer need to free up a slot, or upgrade the ` +
+    `DeepL API plan (the Free plan allows only one glossary per account, ` +
+    `which is not enough for one glossary per domain and direction).`
+  );
+}
+
 async function forwardDeepLError(resp) {
   let detail = '';
   try {