Add test connection feature for database targets

This commit introduces a new endpoint to test database connection. The frontend now includes a button to test the connection before creating a target, with real-time feedback on success or failure. Related styles and components were updated for better user experience.

frontend/src/styles.css

@@ -269,8 +269,8 @@ button {
   border-color: #3384cb;
   background: linear-gradient(180deg, #15528d, #114170);
   box-shadow: inset 0 1px 0 #5f8de144;
-  padding: 7px 12px;
-  min-height: 38px;
+  padding: 6px 12px;
+  min-height: 34px;
 }
 
 .primary-btn:hover {
@@ -278,6 +278,48 @@ button {
   background: linear-gradient(180deg, #1a63a9, #14558f);
 }
 
+.submit-field {
+  align-self: end;
+}
+
+.target-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 8px;
+}
+
+.secondary-btn {
+  font-weight: 600;
+  border-color: #3f6ea9;
+  background: linear-gradient(180deg, #14365f, #102c4f);
+  padding: 6px 12px;
+  min-height: 34px;
+}
+
+.secondary-btn:hover {
+  border-color: #69a9de;
+}
+
+.test-connection-result {
+  margin-top: 10px;
+  font-size: 13px;
+  border-radius: 10px;
+  padding: 8px 10px;
+  border: 1px solid transparent;
+}
+
+.test-connection-result.ok {
+  color: #b9f3cf;
+  border-color: #2f8f63;
+  background: #123727;
+}
+
+.test-connection-result.fail {
+  color: #fecaca;
+  border-color: #b64a4a;
+  background: #3a1c22;
+}
+
 .collapsible {
   padding-top: 12px;
 }