Prevent device sleep during WiFi file transfer and OTA updates

Fixes #199 - device would auto-sleep after 10 minutes while webserver
  was running. Adds preventAutoSleep() method to Activity base class,
  allowing activities to signal they should keep the device awake.

src/activities/Activity.h

@@ -22,4 +22,5 @@ class Activity {
   virtual void onExit() { Serial.printf("[%lu] [ACT] Exiting activity: %s\n", millis(), name.c_str()); }
   virtual void loop() {}
   virtual bool skipLoopDelay() { return false; }
+  virtual bool preventAutoSleep() { return false; }
 };

src/activities/network/CrossPointWebServerActivity.h

@@ -70,4 +70,5 @@ class CrossPointWebServerActivity final : public ActivityWithSubactivity {
   void onExit() override;
   void loop() override;
   bool skipLoopDelay() override { return webServer && webServer->isRunning(); }
+  bool preventAutoSleep() override { return webServer && webServer->isRunning(); }
 };

src/activities/settings/OtaUpdateActivity.h

@@ -41,4 +41,5 @@ class OtaUpdateActivity : public ActivityWithSubactivity {
   void onEnter() override;
   void onExit() override;
   void loop() override;
+  bool preventAutoSleep() override { return state == CHECKING_FOR_UPDATE || state == UPDATE_IN_PROGRESS; }
 };

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
   }