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.

internal/app/openrouter_test.go

@@ -12,7 +12,7 @@ import (
 
 func checkOpenRouterPreview(t *testing.T, a *App, s State, auth <-chan string, key string) {
 	t.Helper()
-	p, err := a.Preview(context.Background(), PreviewRequest{Revision: s.Revision, From: "2026-09-01", To: "2026-09-30", Model: "test/model", Fields: Fields{Category: true}})
+	p, err := runPreview(t, a, PreviewRequest{Revision: s.Revision, From: "2026-09-01", To: "2026-09-30", Model: "test/model", Fields: Fields{Category: true}})
 	if err != nil {
 		t.Fatal(err)
 	}