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/auth/oauth2.go

@@ -14,6 +14,7 @@ import (
 	"slices"
 	"strings"
 	"time"
+	"uuid"
 
 	"gitea.dev/models/db"
 	"gitea.dev/modules/container"
@@ -21,7 +22,6 @@ import (
 	"gitea.dev/modules/timeutil"
 	"gitea.dev/modules/util"
 
-	uuid "github.com/google/uuid"
 	"golang.org/x/crypto/bcrypt"
 	"golang.org/x/oauth2"
 	"xorm.io/builder"