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

docs/new-features.md

@@ -483,9 +483,12 @@ one; paste an index URL or its `owner/repo` and it is remembered in
 A feed author can publish per-mod release stats by adding three optional
 fields to an entry -- `downloads` (total across every release), and
 `first_release` / `last_release` (ISO days) -- which the listing shows in
-the same gold line the MODS tab uses. The fields are additive: feeds that
-carry them stay readable by every build that predates them, and feeds that
-do not render exactly as before.
+the same gold line the MODS tab uses. When a feed does not carry them,
+the row fetches the mod's own GitHub releases instead -- the same cached
+`ModUpdate` fetch the MODS tab uses, one entry per frame -- so the stats
+appear for any mod with a `github` field regardless of feed maintenance.
+The fields are additive: feeds that carry them stay readable by every
+build that predates them, and feeds that do not render exactly as before.
 
 ## Soft reset (all versions)