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/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)