Fix nil division error in exchange rate calculator

Add proper nil handling for tool.current_rate to prevent TypeError when
Synth Finance API fails or returns unsuccessful response

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

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

Author: Shpigford

app/views/tools/widgets/_exchange_rate_calculator.html.erb

@@ -50,7 +50,7 @@
               <img src="https://hatscripts.github.io/circle-flags/flags/<%= tool.to_currency[0..1].downcase %>.svg"
                    alt="<%= tool.to_currency %>"
                    class="w-5 h-5 rounded-full inline-block mr-2">
-              <%= number_to_currency((amount * (tool.current_rate || 0)), unit: tool.to_currency, format: "%n %u") %>
+              <%= number_to_currency(amount * (tool.current_rate || 0), unit: tool.to_currency, format: "%n %u") %>
             </span>
           <% end %>
         <% end %>
@@ -71,7 +71,7 @@
               <img src="https://hatscripts.github.io/circle-flags/flags/<%= tool.from_currency[0..1].downcase %>.svg"
                    alt="<%= tool.from_currency %>"
                    class="w-5 h-5 rounded-full inline-block mr-2">
-              <%= number_to_currency(amount / tool.current_rate, unit: tool.from_currency, format: "%n %u") %>
+              <%= number_to_currency(tool.current_rate ? (amount / tool.current_rate) : 0, unit: tool.from_currency, format: "%n %u") %>
             </span>
           <% end %>
         <% end %>