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

services/attachment/attachment.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"uuid"
 
 	"gitea.dev/models/db"
 	repo_model "gitea.dev/models/repo"
@@ -17,8 +18,6 @@ import (
 	"gitea.dev/modules/storage"
 	"gitea.dev/modules/util"
 	"gitea.dev/services/context/upload"
-
-	"github.com/google/uuid"
 )
 
 // NewAttachment creates a new attachment object, but do not verify.
@@ -85,8 +84,7 @@ func uploadAttachment(ctx context.Context, file *UploaderFile, allowedTypes stri
 	}
 
 	attach, err := NewAttachment(ctx, attach, io.MultiReader(bytes.NewReader(buf), src), file.size)
-	var maxBytesError *http.MaxBytesError
-	if errors.As(err, &maxBytesError) {
+	if _, ok := errors.AsType[*http.MaxBytesError](err); ok {
 		return nil, util.ErrorWrap(util.ErrContentTooLarge, "attachment exceeds limit %d", maxFileSize)
 	}
 	return attach, err