chore: misc go 1.27 tweaks (#39069)

Follow-up to https://github.com/go-gitea/gitea/pull/39068, which
disabled `modernize` entirely.

- re-enable `modernize`, with only the new `embedlit` rule disabled. It
flattens embedded struct literals across ~145 files, and orphans imports
in 6 of them that the fixer does not remove
- apply the rest of the suite: `errors.AsType`, `reflect.TypeAssert`,
`strings.Cut`, and dropping the legacy import comment
- use the new stdlib `uuid` package, `github.com/google/uuid` becomes
indirect
- use `strings.CutLast` in place of manual `LastIndex` slicing in label
scopes, email domains and the diff tree list
- take the header lint skip dirs from the `go.mod` `ignore` directive
and skip dot-directories, instead of hardcoding the list

Assisted-by: Claude Code:claude-opus-5

modules/setting/actions.go

@@ -102,7 +102,7 @@ func loadActionsFrom(rootCfg ConfigProvider) error {
 	}
 
 	if urls := string(Actions.DefaultActionsURL); urls != defaultActionsURLGitHub && urls != defaultActionsURLSelf {
-		url := strings.Split(urls, ",")[0]
+		url, _, _ := strings.Cut(urls, ",")
 		if strings.HasPrefix(url, "https://") || strings.HasPrefix(url, "http://") {
 			log.Error("[actions] DEFAULT_ACTIONS_URL does not support %q as custom URL any longer, fallback to %q",
 				urls,