feat(privacy): reconnect the retroactive cleanup

The privacy filter only decides what gets written from now on. Whatever
was stored before the user narrowed their channels stays there until
something removes it, and that something has had no button since May.

Two rules shape the section, both because this deletes history and
cannot be undone:

- Without a preview the apply button does not exist. Not greyed out,
  absent. A disabled button is something a user waits for; a missing one
  is something they have to go and earn.
- A preview that no longer matches the settings counts as no preview.
  The old version only recoloured the number and left the button live,
  so a changed whitelist could be applied against counts computed for
  the previous one.

The mapping from the live rule to CleanupRetainOnly is the part worth
reading twice. CleanupRetainOnly takes one set and deletes everything
else, so it can only stand in for the live rule where that rule narrows
something: filter off means nothing is filtered, and an empty list means
a full wipe, which has its own button and its own confirmation. Both
cases now say so instead of offering a destructive action that does not
mean what it looks like.

Inside that, the allowlist is the whitelist itself plus any stored
channel this build does not recognise while the unknown-channel failsafe
is on. Deriving it from the counts instead would delete messages that
arrive on a whitelisted but currently empty channel between the preview
and the apply, and dropping the unrecognised ones would defeat the
failsafe, which exists to hold on to a new patch's channel until the
user has decided about it.

The preview runs on a worker over its own connection. It is a GROUP BY
across every stored row, the old version ran it inline on the draw
thread, and holding the read lock for it would stall UpsertMessage on
the framework thread for the length of the scan.

Cleanup_Help_SavedNote stays unused: it tells the reader to press Save
first, and the window it was written for had a Save button.

HellionChat/Resources/HellionStrings.es.resx

@@ -1172,4 +1172,10 @@
   <data name="Settings_Database_Op_Clear" xml:space="preserve">
     <value>borrado del historial</value>
   </data>
+  <data name="Cleanup_Unavailable_FilterOff" xml:space="preserve">
+    <value>El filtro de privacidad está desactivado, así que se guarda cada canal y nada en la base de datos contradice tu configuración. Activa el filtro y elige canales primero.</value>
+  </data>
+  <data name="Cleanup_Unavailable_NothingListed" xml:space="preserve">
+    <value>No hay ningún canal seleccionado, así que una limpieza borraría todo el historial. Elige los canales que quieras conservar, o usa el botón de borrar si de verdad quieres eliminarlo todo.</value>
+  </data>
 </root>