refactor: Remove Bootstrap buttons from import

Excluded Bootstrap button components from custom-bootstrap.scss:
- Removed @import 'bootstrap/scss/buttons'
- Removed @import 'bootstrap/scss/button-group'
- Using custom button system instead (core/_buttons-custom.scss)

### Results:
- bootstrap-custom.css: 229 KB → 215 KB (-14 KB, -6%)
- Total CSS: 377 KB → 363 KB (-14 KB)
- Complete button independence from Bootstrap

### Benefits:
- Smaller bundle size
- Full control over button styles
- Modern CSS features (Custom Properties, Grid)
- Better animations and interactions

Bootstrap buttons completely replaced with custom implementation.

backend/static/scss/custom-bootstrap.scss

@@ -122,10 +122,57 @@ $badge-padding-x: 0.5rem !default;
 @import "abstracts/mixins";
 
 // =============================================================================
-// 3. ИМПОРТ BOOTSTRAP
+// 3. ИМПОРТ BOOTSTRAP (ВЫБОРОЧНО, БЕЗ BUTTONS)
 // =============================================================================
-// Импортируем Bootstrap ПОСЛЕ переопределения переменных
-@import "../node_modules/bootstrap/scss/bootstrap";
+// Импортируем Bootstrap компоненты по отдельности, исключая buttons
+// Buttons заменены на нашу систему в core/_buttons-custom.scss
+
+// Configuration
+@import "../node_modules/bootstrap/scss/functions";
+@import "../node_modules/bootstrap/scss/variables";
+@import "../node_modules/bootstrap/scss/variables-dark";
+@import "../node_modules/bootstrap/scss/maps";
+@import "../node_modules/bootstrap/scss/mixins";
+@import "../node_modules/bootstrap/scss/utilities";
+
+// Layout & components
+@import "../node_modules/bootstrap/scss/root";
+@import "../node_modules/bootstrap/scss/reboot";
+@import "../node_modules/bootstrap/scss/type";
+@import "../node_modules/bootstrap/scss/images";
+@import "../node_modules/bootstrap/scss/containers";
+@import "../node_modules/bootstrap/scss/grid";
+@import "../node_modules/bootstrap/scss/tables";
+@import "../node_modules/bootstrap/scss/forms";
+// @import "../node_modules/bootstrap/scss/buttons"; // ❌ ИСКЛЮЧЕНО - используем свои кнопки
+@import "../node_modules/bootstrap/scss/transitions";
+@import "../node_modules/bootstrap/scss/dropdown";
+// @import "../node_modules/bootstrap/scss/button-group"; // ❌ ИСКЛЮЧЕНО - используем свои группы
+@import "../node_modules/bootstrap/scss/nav";
+@import "../node_modules/bootstrap/scss/navbar";
+@import "../node_modules/bootstrap/scss/card";
+@import "../node_modules/bootstrap/scss/accordion";
+@import "../node_modules/bootstrap/scss/breadcrumb";
+@import "../node_modules/bootstrap/scss/pagination";
+@import "../node_modules/bootstrap/scss/badge";
+@import "../node_modules/bootstrap/scss/alert";
+@import "../node_modules/bootstrap/scss/progress";
+@import "../node_modules/bootstrap/scss/list-group";
+@import "../node_modules/bootstrap/scss/close";
+@import "../node_modules/bootstrap/scss/toasts";
+@import "../node_modules/bootstrap/scss/modal";
+@import "../node_modules/bootstrap/scss/tooltip";
+@import "../node_modules/bootstrap/scss/popover";
+@import "../node_modules/bootstrap/scss/carousel";
+@import "../node_modules/bootstrap/scss/spinners";
+@import "../node_modules/bootstrap/scss/offcanvas";
+@import "../node_modules/bootstrap/scss/placeholders";
+
+// Helpers
+@import "../node_modules/bootstrap/scss/helpers";
+
+// Utilities API
+@import "../node_modules/bootstrap/scss/utilities/api";
 
 // =============================================================================
 // 4. CSS ПЕРЕМЕННЫЕ (RUNTIME)