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

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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/boot_sleep/SleepActivity.cpp

@@ -218,12 +218,12 @@ void SleepActivity::renderCoverSleepScreen() const {
     // Handle XTC file
     Xtc lastXtc(APP_STATE.openEpubPath, "/.crosspoint");
     if (!lastXtc.load()) {
-      Serial.println("[SLP] Failed to load last XTC");
+      Serial.printf("[%lu] [SLP] Failed to load last XTC\n", millis());
       return (this->*renderNoCoverSleepScreen)();
     }
 
     if (!lastXtc.generateCoverBmp()) {
-      Serial.println("[SLP] Failed to generate XTC cover bmp");
+      Serial.printf("[%lu] [SLP] Failed to generate XTC cover bmp\n", millis());
       return (this->*renderNoCoverSleepScreen)();
     }
 
@@ -232,12 +232,12 @@ void SleepActivity::renderCoverSleepScreen() const {
     // Handle TXT file - looks for cover image in the same folder
     Txt lastTxt(APP_STATE.openEpubPath, "/.crosspoint");
     if (!lastTxt.load()) {
-      Serial.println("[SLP] Failed to load last TXT");
+      Serial.printf("[%lu] [SLP] Failed to load last TXT\n", millis());
       return (this->*renderNoCoverSleepScreen)();
     }
 
     if (!lastTxt.generateCoverBmp()) {
-      Serial.println("[SLP] No cover image found for TXT file");
+      Serial.printf("[%lu] [SLP] No cover image found for TXT file\n", millis());
       return (this->*renderNoCoverSleepScreen)();
     }
 
@@ -247,12 +247,12 @@ void SleepActivity::renderCoverSleepScreen() const {
     Epub lastEpub(APP_STATE.openEpubPath, "/.crosspoint");
     // Skip loading css since we only need metadata here
     if (!lastEpub.load(true, true)) {
-      Serial.println("[SLP] Failed to load last epub");
+      Serial.printf("[%lu] [SLP] Failed to load last epub\n", millis());
       return (this->*renderNoCoverSleepScreen)();
     }
 
     if (!lastEpub.generateCoverBmp(cropped)) {
-      Serial.println("[SLP] Failed to generate cover bmp");
+      Serial.printf("[%lu] [SLP] Failed to generate cover bmp\n", millis());
       return (this->*renderNoCoverSleepScreen)();
     }
 
@@ -265,7 +265,7 @@ void SleepActivity::renderCoverSleepScreen() const {
   if (Storage.openFileForRead("SLP", coverBmpPath, file)) {
     Bitmap bitmap(file);
     if (bitmap.parseHeaders() == BmpReaderError::Ok) {
-      Serial.printf("[SLP] Rendering sleep cover: %s\n", coverBmpPath.c_str());
+      Serial.printf("[%lu] [SLP] Rendering sleep cover: %s\n", millis(), coverBmpPath.c_str());
       renderBitmapSleepScreen(bitmap);
       return;
     }

src/activities/home/RecentBooksActivity.cpp

@@ -73,7 +73,7 @@ void RecentBooksActivity::loop() {
 
   if (mappedInput.wasReleased(MappedInputManager::Button::Confirm)) {
     if (!recentBooks.empty() && selectorIndex < static_cast<int>(recentBooks.size())) {
-      Serial.printf("Selected recent book: %s\n", recentBooks[selectorIndex].path.c_str());
+      Serial.printf("[%lu] [RBA] Selected recent book: %s\n", millis(), recentBooks[selectorIndex].path.c_str());
       onSelectBook(recentBooks[selectorIndex].path);
       return;
     }

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,