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/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; }
 };