fix: Manually trigger GPIO update in File Browser mode (#819)

## Summary

* Manually trigger GPIO update in File Browser mode
* Previously just assumed that the GPIO data would update automatically
(presumably via yield), the data is currently updated in the main loop
(and now here as well during the middle of the processing loop).
* This allows the back button to be correctly detected instead of only
being checked once every 100ms or so for the button state.

## Additional Context

* Fixes
https://github.com/crosspoint-reader/crosspoint-reader/issues/579

---

### AI Usage

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Did you use AI tools to help write this code? No


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Enhanced input state detection in the web server interface for more
responsive and accurate user command recognition during high-frequency
operations.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/MappedInputManager.h

@@ -15,6 +15,7 @@ class MappedInputManager {
 
   explicit MappedInputManager(HalGPIO& gpio) : gpio(gpio) {}
 
+  void update() const { gpio.update(); }
   bool wasPressed(Button button) const;
   bool wasReleased(Button button) const;
   bool isPressed(Button button) const;

src/activities/network/CrossPointWebServerActivity.cpp

@@ -348,6 +348,9 @@ void CrossPointWebServerActivity::loop() {
         // Yield and check for exit button every 64 iterations
         if ((i & 0x3F) == 0x3F) {
           yield();
+          // Force trigger an update of which buttons are being pressed so be have accurate state
+          // for back button checking
+          mappedInput.update();
           // Check for exit button inside loop for responsiveness
           if (mappedInput.wasPressed(MappedInputManager::Button::Back)) {
             onGoBack();