add sample file downloads in each upload card

frontend/index.html

@@ -424,6 +424,16 @@ input[type="range"]::-webkit-slider-thumb {
   animation: indeterminate 1.4s ease infinite;
 }
 
+.sample-link {
+  display: inline-block;
+  margin-top: 0.6rem;
+  font-size: 13px;
+  color: var(--teal);
+  text-decoration: none;
+  font-weight: 500;
+}
+.sample-link:hover { color: var(--teal-d); text-decoration: underline; }
+
 .hidden { display: none !important; }
 ::-webkit-scrollbar { width: 5px; }
 ::-webkit-scrollbar-track { background: transparent; }
@@ -484,6 +494,7 @@ input[type="range"]::-webkit-slider-thumb {
         </div>
       </div>
       <div class="file-chips" id="chipsModel"></div>
+      <a href="/sample_model.xlsx" download class="sample-link">⬇ Download sample file</a>
     </div>
 
     <!-- Bottom-left: Step 2 -->
@@ -503,6 +514,7 @@ input[type="range"]::-webkit-slider-thumb {
       </div>
       <div class="file-chips" id="chipsMapping"></div>
       <div class="mismatch-warning hidden" id="prefixWarning"></div>
+      <a href="/sample_mapping.xlsx" download class="sample-link">⬇ Download sample file</a>
     </div>
 
     <!-- Bottom-right: Step 3 -->
@@ -521,6 +533,7 @@ input[type="range"]::-webkit-slider-thumb {
         </div>
       </div>
       <div class="file-chips" id="chipsExpr"></div>
+      <a href="/sample_combined_geneExpr.csv" download class="sample-link">⬇ Download sample file</a>
     </div>
 
   </div><!-- /main-grid -->

frontend/sample_combined_geneExpr.csv

@@ -0,0 +1,4 @@
+gene_id,sample_1,sample_2
+NCBI_001,12.5,8.1
+NCBI_002,0.0,3.2
+NCBI_003,7.3,0.0