feat: Add visual clear form tip with OS detection

Added visible tip in interface showing correct keyboard shortcut:
- Automatically detects Mac vs Windows/Linux
- Shows 'Cmd + Shift + R' on Mac
- Shows 'Ctrl + Shift + R' on Windows/Linux
- Clean, non-intrusive design with blue accent
- Positioned below 'Generar Venta' button

Technical implementation:
- CSS styling for clear-form-tip class
- JavaScript OS detection using navigator.platform
- Dynamic content update based on detected OS
- Updated cache-busting version for immediate loading

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

styles.css

@@ -1927,6 +1927,34 @@ table tbody tr:hover {
     font-size: 12px;
 }
 
+/* Tip para limpiar formulario */
+.clear-form-tip {
+    text-align: center;
+    margin-top: 12px;
+    padding: 8px 12px;
+    background: #f8f9fa;
+    border-left: 3px solid #007bff;
+    border-radius: 4px;
+    color: #495057;
+    font-size: 12px;
+}
+
+.clear-form-tip strong {
+    color: #212529;
+}
+
+#clear-shortcut {
+    display: inline-block;
+    background: #e9ecef;
+    padding: 2px 6px;
+    border-radius: 3px;
+    font-family: 'Courier New', monospace;
+    font-weight: bold;
+    color: #343a40;
+    border: 1px solid #ced4da;
+    margin-left: 4px;
+}
+
 /* --- Checkout Panel --- */
 .checkout-panel {
     background: #fff;