feat: Add comprehensive validation for expense submission fields in bottom sheet

lib/model/expense/add_expense_bottom_sheet.dart

@@ -118,7 +118,61 @@ class _AddExpenseBottomSheetState extends State<_AddExpenseBottomSheet> {
           onCancel: Get.back,
           onSubmit: () {
             if (_formKey.currentState!.validate()) {
+              // Additional dropdown validation
+              if (controller.selectedProject.value.isEmpty) {
+                showAppSnackbar(
+                  title: "Error",
+                  message: "Please select a project",
+                  type: SnackbarType.error,
+                );
+                return;
+              }
+
+              if (controller.selectedExpenseType.value == null) {
+                showAppSnackbar(
+                  title: "Error",
+                  message: "Please select an expense type",
+                  type: SnackbarType.error,
+                );
+                return;
+              }
+
+              if (controller.selectedPaymentMode.value == null) {
+                showAppSnackbar(
+                  title: "Error",
+                  message: "Please select a payment mode",
+                  type: SnackbarType.error,
+                );
+                return;
+              }
+
+              if (controller.selectedPaidBy.value == null) {
+                showAppSnackbar(
+                  title: "Error",
+                  message: "Please select a person who paid",
+                  type: SnackbarType.error,
+                );
+                return;
+              }
+
+              if (controller.attachments.isEmpty &&
+                  controller.existingAttachments.isEmpty) {
+                showAppSnackbar(
+                  title: "Error",
+                  message: "Please attach at least one document",
+                  type: SnackbarType.error,
+                );
+                return;
+              }
+
+              // Validation passed, submit
               controller.submitOrUpdateExpense();
+            } else {
+              showAppSnackbar(
+                title: "Error",
+                message: "Please fill all required fields correctly",
+                type: SnackbarType.error,
+              );
             }
           },
           child: SingleChildScrollView(
@@ -186,12 +240,6 @@ class _AddExpenseBottomSheetState extends State<_AddExpenseBottomSheet> {
                 _CustomTextField(
                   controller: controller.gstController,
                   hint: "Enter GST No.",
-                  validator: (value) {
-                    if (value != null && value.isNotEmpty) {
-                      return Validators.gstValidator(value);
-                    }
-                    return null;
-                  },
                 ),
 
                 MySpacing.height(16),
@@ -284,7 +332,7 @@ class _AddExpenseBottomSheetState extends State<_AddExpenseBottomSheet> {
                     if (value != null && value.isNotEmpty) {
                       return Validators.transactionIdValidator(value);
                     }
-                    return null; 
+                    return null;
                   },
                 ),