Prevent device sleep during WiFi file transfer and OTA updates (#203)

## Summary

* **What is the goal of this PR?** Fixes #199 - Device falls asleep
during WiFi file transfer after 10 minutes of inactivity, disconnecting
the web server.

  * **What changes are included?**
    - Add `preventAutoSleep()` virtual method to `Activity` base class
- Modify main loop to reset inactivity timer when `preventAutoSleep()`
returns true
- Override `preventAutoSleep()` in `CrossPointWebServerActivity`
(returns true when web server running)
- Override `preventAutoSleep()` in `OtaUpdateActivity` (returns true
during update check/download)

  ## Additional Context

* The existing `skipLoopDelay()` method controls loop timing (yield vs
delay) for HTTP responsiveness. The new `preventAutoSleep()` method is
semantically separate - it explicitly signals that an activity should
keep the device awake.
* `CrossPointWebServerActivity` uses both methods: `skipLoopDelay()` for
responsive HTTP handling, `preventAutoSleep()` for staying awake.
* `OtaUpdateActivity` only needs `preventAutoSleep()` since the OTA
library handles HTTP internally.

src/main.cpp

@@ -316,9 +316,10 @@ void loop() {
     lastMemPrint = millis();
   }
 
-  // Check for any user activity (button press or release)
+  // Check for any user activity (button press or release) or active background work
   static unsigned long lastActivityTime = millis();
-  if (inputManager.wasAnyPressed() || inputManager.wasAnyReleased()) {
+  if (inputManager.wasAnyPressed() || inputManager.wasAnyReleased() ||
+      (currentActivity && currentActivity->preventAutoSleep())) {
     lastActivityTime = millis();  // Reset inactivity timer
   }