Run classification previews in the background with live progress

The preview endpoint held one HTTP request open while classifying
serially at three-second pacing, so any real range meant minutes of a
grayed-out button and per-row errors were invisible until the loop
ended. Analyse now starts a single background run against its own
snapshot; a progress endpoint reports analysed counts, proposed
changes and errors as they happen, and the page polls it with a
progress bar, pace-based estimate and a Stop button. Navigating away
no longer orphans the run: the page re-attaches to it on return.

A run that has produced no successful proposal and fails three times
in a row with the identical error stops early and reports that error,
so a wrong key or unsupported model surfaces in seconds instead of
repeating across the whole paced range.

Also normalize a null settings.private_names, which crashed the whole
UI on a workspace that had never saved preferences.

web/src/styles.css

@@ -1389,6 +1389,18 @@ summary .badge {
     transform: rotate(360deg);
   }
 }
+.progress-track {
+  height: 8px;
+  border-radius: 4px;
+  background: #e1e9e5;
+  overflow: hidden;
+}
+.progress-fill {
+  height: 100%;
+  border-radius: 4px;
+  background: var(--emerald);
+  transition: width 0.6s ease;
+}
 footer {
   display: flex;
   justify-content: space-between;