fix: show backend error detail in context-popup compact button (#2721)

When the context-popup compact button receives a non-OK response (e.g.
409 for active-run), the error detail from the backend was being
discarded in favor of a generic 'Compaction failed' message.

Now parses the JSON response body for non-OK responses and prefers the
detail field when present, matching the behavior of the /compact slash
command. Uses textContent for safe rendering.

Co-authored-by: michaelxer <michaelxer@users.noreply.github.com>

static/js/chatRenderer.js

@@ -1877,7 +1877,13 @@ export function displayMetrics(messageElement, metrics) {
                 }
               }, 200);
             } else {
-              compactBody.innerHTML = '<span style="color:var(--red);">Compaction failed. Try again later.</span>';
+              let detail = 'Compaction failed. Try again later.';
+              try {
+                const err = await res.json();
+                if (err.detail) detail = err.detail;
+              } catch {}
+              compactBody.textContent = detail;
+              compactBody.style.color = 'var(--red)';
             }
           } catch (err) {
             clearInterval(waveInterval);