Show mod downloads and release dates in the launcher MODS and Find Mods panels (#793)

* Resolve Find Mods stats from each mod's GitHub repo when the feed lacks them

A FIND MODS row now shows download/date stats even when its feed publishes
none: the row fetches the mod's own GitHub releases through the same
cached ModUpdate.fetchReleases the MODS tab uses (six-hour options cache,
so an installed mod's repo is instant). Feed-published stats still win
when present; otherwise one repo is fetched per frame -- the thumbnail
budget pattern -- so opening the tab never stalls for the whole listing.
ModUpdate.statsForReleases is the shared resolver.

* Fix crash opening the Find Mods tab: rename the stats cache field

The resolver stored results in self._findStats, which collides with the
method of the same name: self._findStats resolves through the metatable to
the function, so the or {} guard never fired and indexing it crashed the
launcher the moment the panel built. State now lives in _findStatsCache.

* Fix Find Mods crash: require ModUpdate in the find panel

buildFindPanel called ModUpdate.statsLine without a local require --
only buildModsPanel had one -- so opening the tab indexed a nil global.

* Retry Find Mods stats after failed repo fetches

A failed repo fetch (hourly GitHub API rate limit, transient network error)
was memoized as resolved, so a rate-limited first visit left those rows
empty for the whole session. Failures now schedule a 60s retry; a 404 is
still permanent so a renamed or vanished repo is fetched once.

* Add the MODS tab sort options to the Find Mods tab

src/mods/ModUpdate.lua

@@ -229,6 +229,20 @@ function ModUpdate.releaseDates(releases)
   return { first = first, latest = latest }
 end
 
+-- One resolver over a release list: { total, first, latest } or nil when
+-- the list carries neither counts nor dates.  The FIND MODS rows use this
+-- on the repo's fetched releases, the same source the MODS tab trusts.
+function ModUpdate.statsForReleases(releases)
+  local dl = ModUpdate.totalDownloads(releases)
+  local d = ModUpdate.releaseDates(releases)
+  if not dl and not d then return nil end
+  return {
+    total = dl and dl.total or nil,
+    first = d and d.first or nil,
+    latest = d and d.latest or nil,
+  }
+end
+
 -- Thousands-separated count for the launcher ("12,345"), plain for small
 -- numbers.  Never throws; garbage in, "0" out.
 function ModUpdate.formatCount(n)