Fix `recentupdate` sorting bugs (#32505)

Fix #32499

- Add the missing `recentupdate` to `OrderByFlatMap`
- Assign default value(`recentupdate`) to `EXPLORE_PAGING_DEFAULT_SORT`

models/repo/search.go

@@ -36,6 +36,7 @@ var OrderByMap = map[string]map[string]db.SearchOrderBy{
 var OrderByFlatMap = map[string]db.SearchOrderBy{
 	"newest":                OrderByMap["desc"]["created"],
 	"oldest":                OrderByMap["asc"]["created"],
+	"recentupdate":          OrderByMap["desc"]["updated"],
 	"leastupdate":           OrderByMap["asc"]["updated"],
 	"reversealphabetically": OrderByMap["desc"]["alpha"],
 	"alphabetically":        OrderByMap["asc"]["alpha"],

modules/setting/ui.go

@@ -86,6 +86,7 @@ var UI = struct {
 	Reactions:               []string{`+1`, `-1`, `laugh`, `hooray`, `confused`, `heart`, `rocket`, `eyes`},
 	CustomEmojis:            []string{`git`, `gitea`, `codeberg`, `gitlab`, `github`, `gogs`},
 	CustomEmojisMap:         map[string]string{"git": ":git:", "gitea": ":gitea:", "codeberg": ":codeberg:", "gitlab": ":gitlab:", "github": ":github:", "gogs": ":gogs:"},
+	ExploreDefaultSort:      "recentupdate",
 	PreferredTimestampTense: "mixed",
 
 	AmbiguousUnicodeDetection: true,