## 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.
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