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

models/issues/label.go

@@ -186,11 +186,11 @@ func (l *Label) ExclusiveScope() string {
 	if !l.Exclusive {
 		return ""
 	}
-	lastIndex := strings.LastIndex(l.Name, "/")
-	if lastIndex == -1 || lastIndex == 0 || lastIndex == len(l.Name)-1 {
+	scope, name, found := strings.CutLast(l.Name, "/")
+	if !found || scope == "" || name == "" {
 		return ""
 	}
-	return l.Name[:lastIndex]
+	return scope
 }
 
 // CompareLabelForDisplay compares labels for displaying them in dropdowns or lists.