s3: give account-less identities a distinct owner instead of admin (#9962)

* s3: stop collapsing account-less identities into the admin account

Identities configured without an account block all defaulted to the
shared admin account, so distinct users got the same owner id and
ownership checks could not tell them apart. checkAccessByOwnership also
treated that id as an admin bypass, so any account-less caller passed
ownership for any bucket. Give such identities a distinct account id from
their name, and decide the ownership admin bypass by Admin capability
rather than by the account id. isUserAdmin is now nil-safe.

* s3: use the context identity in isUserAdmin before re-authenticating

The Auth middleware already verifies and stores the identity in the
request context. Read it there first so the ownership/admin checks don't
re-run signature verification, which is redundant and fails once the
request body has been consumed.

* s3: nil-guard the context identity in isUserAdmin

A non-nil interface wrapping a typed-nil *Identity passes the type
assertion; guard against it before calling isAdmin().

* s3: trim verbose comments

weed/s3api/auth_credentials.go

@@ -137,6 +137,19 @@ var (
 	}
 )
 
+// accountForUnscopedIdentity gives an identity with no configured account a
+// distinct account id from its name, so account-less identities are not all
+// collapsed into the shared admin account for ownership/ACL checks.
+func accountForUnscopedIdentity(name string) *Account {
+	if name == "" || name == AccountAdmin.Id {
+		return &AccountAdmin
+	}
+	return &Account{
+		Id:          name,
+		DisplayName: name,
+	}
+}
+
 type Credential struct {
 	AccessKey  string
 	SecretKey  string
@@ -614,7 +627,7 @@ func (iam *IdentityAccessManagement) ReplaceS3ApiConfiguration(config *iam_pb.S3
 			t.Account = &AccountAnonymous
 			identityAnonymous = t
 		case ident.Account == nil:
-			t.Account = &AccountAdmin
+			t.Account = accountForUnscopedIdentity(t.Name)
 		default:
 			if account, ok := accounts[ident.Account.Id]; ok {
 				t.Account = account
@@ -833,7 +846,7 @@ func (iam *IdentityAccessManagement) MergeS3ApiConfiguration(config *iam_pb.S3Ap
 			t.Account = &AccountAnonymous
 			identityAnonymous = t
 		case ident.Account == nil:
-			t.Account = &AccountAdmin
+			t.Account = accountForUnscopedIdentity(t.Name)
 		default:
 			if account, ok := accounts[ident.Account.Id]; ok {
 				t.Account = account