fix: Prevent sleeping when in OPDS browser / downloading books (#818)

## Summary

* Prevent sleeping when in OPDS browser / downloading books

## Additional Context

* Raised in
https://github.com/crosspoint-reader/crosspoint-reader/discussions/673

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### AI Usage

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Did you use AI tools to help write this code? No

src/activities/browser/OpdsBookBrowserActivity.h

@@ -64,4 +64,5 @@ class OpdsBookBrowserActivity final : public ActivityWithSubactivity {
   void navigateToEntry(const OpdsEntry& entry);
   void navigateBack();
   void downloadBook(const OpdsEntry& book);
+  bool preventAutoSleep() override { return true; }
 };