fix: Specify default value for `EXPLORE_DEFAULT_SORT`.

- This is another regression from
5a0bc35799, where the default value was
changed to "alphabetically" because it relied on `ExploreDefaultSort`
providing a fallback value.
- Set the default value for `EXPLORE_DEFAULT_SORT` to `recentupdate`,
this was already the behavior explicitly for existing users of this setting
but with 5a0bc35799 it didn't provide a
explicit fallback to `recentupdate`. So opting for a 'easy' fix, that
doesn't add boilerplate code to those instances.

(cherry picked from commit f4be4e733c)

modules/setting/ui.go

@@ -88,6 +88,7 @@ var UI = struct {
 	Reactions:               []string{`+1`, `-1`, `laugh`, `hooray`, `confused`, `heart`, `rocket`, `eyes`},
 	CustomEmojis:            []string{`git`, `gitea`, `codeberg`, `gitlab`, `github`, `gogs`, `forgejo`},
 	CustomEmojisMap:         map[string]string{"git": ":git:", "gitea": ":gitea:", "codeberg": ":codeberg:", "gitlab": ":gitlab:", "github": ":github:", "gogs": ":gogs:", "forgejo": ":forgejo:"},
+	ExploreDefaultSort:      "recentupdate",
 	PreferredTimestampTense: "mixed",
 
 	AmbiguousUnicodeDetection: true,