feat(settings): scan from Settings, and watch every job in place

Scanning goes back where libraries are managed -- the Jobs tab's own
comment said the per-library controls had been taken out of Settings to
build it. "Scan All" and "Full Rescan" join "Add Library", "Scan now"
joins the per-library overflow menu beside Rename and Remove, and a
library being scanned says so where its track count goes.

Each surface then gets the job rows for its own kind: scans under
Libraries, index and enrichment under Search Index, downloads under the
download clients, and the autotag apply in the Autotag view -- where
stopping a run matters most, since applying rewrites tags on disk and
that view had no cancel at all.

The autotag panel shares the header's grid row through a wrapper rather
than taking a third row: it is display:none while nothing is applying,
which is nearly always, and a grid row would still spend the
container's gap on it.

The Launch Page select is derived from `VIEW_META` instead of listing
its ten options, since removing a destination is exactly the change
that leaves two hand-written copies disagreeing.

frontend/src/components/download-picker/download-picker.ts

@@ -246,7 +246,7 @@ export class DownloadPicker extends LitElement {
             return html`
                 <wa-callout variant="success">
                     Found a clear match and started downloading it. Progress is
-                    in the background jobs panel.
+                    on the Downloads page.
                 </wa-callout>
             `;
         }