fix(s3): use URL-safe secret keys for dashboard users and service accounts (#8902)

* fix(s3): use URL-safe secret keys for admin dashboard users and service accounts

The dashboard's generateSecretKey() used base64.StdEncoding which produces
+, /, and = characters that break S3 signature authentication. Reuse the
IAM package's GenerateSecretAccessKey() which was already fixed in #7990.

Fixes #8898

* fix: handle error from GenerateSecretAccessKey instead of ignoring it

weed/admin/dash/user_management_test.go

@@ -74,9 +74,16 @@ func TestGenerateSecretKey(t *testing.T) {
 	key1 := generateSecretKey()
 	key2 := generateSecretKey()
 
-	// Check length (base64 encoding of 30 bytes = 40 characters)
-	if len(key1) != 40 {
-		t.Errorf("Expected secret key length 40, got %d", len(key1))
+	// Check length (IAM standard secret key length)
+	if len(key1) != 42 {
+		t.Errorf("Expected secret key length 42, got %d", len(key1))
+	}
+
+	// Check that key contains only URL-safe characters (no +, /)
+	for _, c := range key1 {
+		if c == '+' || c == '/' || c == '=' {
+			t.Errorf("Secret key contains non-URL-safe character: %c", c)
+		}
 	}
 
 	// Check uniqueness