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/git/commit.go

@@ -114,8 +114,7 @@ func (c *Commit) HasPreviousCommit(ctx context.Context, gitRepo *Repository, obj
 	if err == nil {
 		return true, nil
 	}
-	var exitError *exec.ExitError
-	if errors.As(err, &exitError) {
+	if exitError, ok := errors.AsType[*exec.ExitError](err); ok {
 		if exitError.ProcessState.ExitCode() == 1 && len(exitError.Stderr) == 0 {
 			return false, nil
 		}