fix: chore: make all debug messages uniform (#825)

## Summary

* Unify all serial port debug messages

## Additional Context

* All messages sent to the serial port now follow the "[timestamp]
[origin] payload" format (notable exception framework messages)

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

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

src/RecentBooksStore.cpp

@@ -83,7 +83,7 @@ RecentBook RecentBooksStore::getDataFromBook(std::string path) const {
     lastBookFileName = path.substr(lastSlash + 1);
   }
 
-  Serial.printf("Loading recent book: %s\n", path.c_str());
+  Serial.printf("[%lu] [RBS] Loading recent book: %s\n", millis(), path.c_str());
 
   // If epub, try to load the metadata for title/author and cover
   if (StringUtils::checkFileExtension(lastBookFileName, ".epub")) {