feat: implement landing page with file upload, sample loading, and comprehensive JSON schema documentation

frontend/src/index.css

@@ -275,6 +275,36 @@ input, textarea, select { font-family: inherit; font-size: inherit; color: inher
   font-size: 13.5px;
 }
 
+.download-instructions-btn {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  margin-top: 12px;
+  width: 100%;
+  justify-content: center;
+  font-size: 12.5px;
+  color: var(--ink-3);
+  padding: 10px 14px;
+  border-radius: var(--r-md);
+  transition: all 150ms ease;
+}
+.download-instructions-btn:hover {
+  color: var(--accent);
+  background: var(--accent-weak);
+}
+.download-instructions-icon {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  width: 20px;
+  height: 20px;
+  border: 1.5px solid currentColor;
+  border-radius: 5px;
+  font-size: 13px;
+  line-height: 1;
+  flex-shrink: 0;
+}
+
 .samples h4 {
   font-size: 11px;
   font-family: var(--font-mono);