feat: Add MAC address display to WiFi Networks screen (#381)

## Summary

* Implements #380, allowing the user to see the device's MAC address in
order to register on wifi networks

## Additional Context

* Although @markatlnk suggested showing on the settings screen, I
implemented display at the bottom of the WiFi Networks selection screen
(accessed via "File Transfer" > "Join a Network") since I think it makes
more sense there.
* Tested on my own device


![IMG_2873](https://github.com/user-attachments/assets/b82a20dc-41a0-4b21-81f1-20876aa2c6b0)


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### AI Usage

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Did you use AI tools to help write this code? _**YES**_

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/activities/network/WifiSelectionActivity.h

@@ -62,6 +62,9 @@ class WifiSelectionActivity final : public ActivityWithSubactivity {
   // Password to potentially save (from keyboard or saved credentials)
   std::string enteredPassword;
 
+  // Cached MAC address string for display
+  std::string cachedMacAddress;
+
   // Whether network was connected using a saved password (skip save prompt)
   bool usedSavedPassword = false;