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/validation/helpers.go

@@ -64,12 +64,12 @@ func IsEmailDomainListed(globs []glob.Glob, email string) bool {
 		return false
 	}
 
-	n := strings.LastIndex(email, "@")
-	if n <= 0 {
+	localPart, domain, found := strings.CutLast(email, "@")
+	if !found || localPart == "" {
 		return false
 	}
 
-	domain := strings.ToLower(email[n+1:])
+	domain = strings.ToLower(domain)
 
 	for _, g := range globs {
 		if g.Match(domain) {