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/log/logger_impl.go

@@ -184,7 +184,7 @@ func asLogStringer(v any) LogStringer {
 		// in case the receiver is a pointer, but the value is a struct
 		vp := reflect.New(a.Type())
 		vp.Elem().Set(a)
-		if s, ok := vp.Interface().(LogStringer); ok {
+		if s, ok := reflect.TypeAssert[LogStringer](vp); ok {
 			return s
 		}
 	}