feat(front-end): upload only supported file and add step warning message

frontend/src/app/features/calculator/components/upload-form/upload-form.component.html

@@ -3,10 +3,16 @@
   <div class="section">
     @if (selectedFile()) {
       <div class="viewer-wrapper">
-         <app-stl-viewer 
-            [file]="selectedFile()"
-            [color]="getSelectedFileColor()">
-         </app-stl-viewer>
+         @if (isStepFile(selectedFile())) {
+             <div class="step-warning">
+                 <p>{{ 'CALC.STEP_WARNING' | translate }}</p>
+             </div>
+         } @else {
+             <app-stl-viewer 
+                [file]="selectedFile()"
+                [color]="getSelectedFileColor()">
+             </app-stl-viewer>
+         }
          <!-- Close button removed as requested -->
       </div>
     }

frontend/src/app/features/calculator/components/upload-form/upload-form.component.scss

@@ -202,6 +202,18 @@
 .progress-fill {
     height: 100%;
     background: var(--color-brand);
-    width: 0%;
-    transition: width 0.2s ease-out;
+}
+
+.step-warning {
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    height: 300px;
+    background: var(--color-neutral-100);
+    border: 1px dashed var(--color-border);
+    border-radius: var(--radius-md);
+    padding: var(--space-4);
+    text-align: center;
+    color: var(--color-text-muted);
+    font-weight: 500;
 }

frontend/src/app/features/calculator/components/upload-form/upload-form.component.ts

@@ -61,7 +61,13 @@ export class UploadFormComponent implements OnInit {
       }
   }
 
-  acceptedFormats = '.stl,.3mf,.step,.stp,.obj,.amf,.ply,.igs,.iges';
+  acceptedFormats = '.stl,.3mf,.step,.stp';
+
+  isStepFile(file: File | null): boolean {
+      if (!file) return false;
+      const name = file.name.toLowerCase();
+      return name.endsWith('.step') || name.endsWith('.stp');
+  }
 
   constructor() {
     this.form = this.fb.group({