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/app.go

@@ -71,6 +71,7 @@ type App struct {
 	bank            banking.Provider
 	classifier      classification.Client
 	previews        map[string]Preview
+	previewRun      *previewJob
 	taxonomies      map[string]TaxonomyPreview
 	csvImports      map[string]CSVImport
 	authStates      map[string]authorization