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/activities/reader/EpubReaderActivity.cpp

@@ -664,9 +664,9 @@ void EpubReaderActivity::saveProgress(int spineIndex, int currentPage, int pageC
     data[5] = (pageCount >> 8) & 0xFF;
     f.write(data, 6);
     f.close();
-    Serial.printf("[ERS] Progress saved: Chapter %d, Page %d\n", spineIndex, currentPage);
+    Serial.printf("[%lu] [ERS] Progress saved: Chapter %d, Page %d\n", millis(), spineIndex, currentPage);
   } else {
-    Serial.printf("[ERS] Could not save progress!\n");
+    Serial.printf("[%lu] [ERS] Could not save progress!\n", millis());
   }
 }
 void EpubReaderActivity::renderContents(std::unique_ptr<Page> page, const int orientedMarginTop,