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>

index.html

@@ -343,6 +343,13 @@
                   <span>Generar Venta</span>
                 </button>
               </div>
+              
+              <!-- Tip para limpiar formulario -->
+              <div class="clear-form-tip">
+                <small>💡 <strong>Para limpiar:</strong> 
+                  <span id="clear-shortcut">Ctrl + Shift + R</span>
+                </small>
+              </div>
             </form>
 
           </div> <!-- fin checkout-panel -->
@@ -768,6 +775,6 @@
   <div id="printArea" class="no-print"></div>
 
   <script src="https://cdn.jsdelivr.net/npm/qrcode@1/build/qrcode.min.js"></script>
-  <script type="module" src="app.js?v=1757462000"></script>
+  <script type="module" src="app.js?v=1757470000"></script>
 </body>
 </html>