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

routers/api/actions/artifacts_utils.go

@@ -83,7 +83,7 @@ func parseArtifactItemPath(ctx *ArtifactContext) (string, string, bool) {
 	// it's formatted as {artifact_name}/{artfict_path_in_runner}
 	// runner in host mode on Windows, itemPath is joined by Windows slash '\'
 	itemPath := util.PathJoinRelX(ctx.Req.URL.Query().Get("itemPath"))
-	artifactName := strings.Split(itemPath, "/")[0]
+	artifactName, _, _ := strings.Cut(itemPath, "/")
 	artifactPath := strings.TrimPrefix(itemPath, artifactName+"/")
 	if !validateArtifactHash(ctx, artifactName) {
 		return "", "", false

routers/api/actions/runner/runner.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"net/http"
 	"slices"
+	"uuid"
 
 	runnerv1 "gitea.dev/actionslib/runner/v1"
 	"gitea.dev/actionslib/runner/v1/runnerv1connect"
@@ -20,7 +21,6 @@ import (
 	actions_service "gitea.dev/services/actions"
 
 	"connectrpc.com/connect"
-	gouuid "github.com/google/uuid"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
 	"google.golang.org/protobuf/proto"
@@ -74,7 +74,7 @@ func (s *Service) Register(
 	// create new runner
 	name := util.EllipsisDisplayString(req.Msg.Name, 255)
 	runner := &actions_model.ActionRunner{
-		UUID:                 gouuid.New().String(),
+		UUID:                 uuid.New().String(),
 		Name:                 name,
 		OwnerID:              runnerToken.OwnerID,
 		RepoID:               runnerToken.RepoID,

routers/api/packages/container/container.go

@@ -598,8 +598,7 @@ func PutManifest(ctx *context.Context) {
 
 	digest, err := processManifest(ctx, mci, buf)
 	if err != nil {
-		var namedError *namedError
-		if errors.As(err, &namedError) {
+		if namedError, ok := errors.AsType[*namedError](err); ok {
 			apiErrorDefined(ctx, namedError)
 		} else if errors.Is(err, container_model.ErrContainerBlobNotExist) {
 			apiErrorDefined(ctx, errBlobUnknown)