fix(settings): show which channels get their history deleted

Three of the four first-run profiles switch the retention sweep on and write a
per-channel deletion policy. The window only ever showed the global default, so
whatever the wizard decided about individual channels was invisible from the
moment the wizard closed.

The tab now lists every channel whose retention differs from the global value,
tagged by where the number comes from, plus a button to drop the custom ones. It
also finally uses the strings written for this screen, sweep description and
default help included.

Read-only, deliberately. Zero means "keep forever" as a global default and
"delete this channel's entire history" as a per-channel value, because the
default takes a separate SQL branch while a mapped channel gets cutoff = now. No
shipped profile contains a zero and no UI could set one, so nothing is broken
today -- but an editor cannot be offered until those two meanings agree. Noted
at the branch in DeleteByRetentionPolicy.

The default slider now goes down to 0, which is the value that means "never
delete anything". Its own label has been promising "0 = never" while the slider
started at 1.

HellionChat/MessageStore.cs

@@ -529,6 +529,12 @@ internal class MessageStore : IDisposable
                 var index = 0;
                 foreach (var (type, days) in chatTypeDaysMap)
                 {
+                    // Careful: 0 here is NOT the "keep forever" it means for
+                    // defaultDays below. A per-channel 0 puts the cutoff at now
+                    // and deletes the channel's entire history. No profile ships
+                    // a 0 and no UI can set one, which is why this is a comment
+                    // and not a guard -- but any editor added later has to
+                    // reconcile the two meanings before it exposes the value.
                     var cutoff = nowMs - days * 86400000L;
                     var typeParam = $"$type{index}";
                     var cutoffParam = $"$cutoff{index}";