refactor: Removed SdCardFontGlobals.h (#1962)

## Summary

First of several changes to decouple and clean up SD card fonts
integration. This change eliminates SdCardFontGlobals.h:
- Simply declare the `extern SdCardFontSystem sdFontSystem` in
SdCardFontSystem.h.
- `ActivityManager::goToReader` should not care about loading SD card
fonts. Instead do the same work in `ReaderActivity::onEnter`, after the
previous activity has exited and after ReaderActivity has validated the
file path.

---

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src/activities/reader/ReaderActivity.cpp

@@ -6,6 +6,7 @@
 #include "CrossPointSettings.h"
 #include "Epub.h"
 #include "EpubReaderActivity.h"
+#include "SdCardFontSystem.h"
 #include "Txt.h"
 #include "TxtReaderActivity.h"
 #include "Xtc.h"
@@ -103,6 +104,8 @@ void ReaderActivity::onEnter() {
     return;
   }
 
+  sdFontSystem.ensureLoaded(renderer);
+
   currentBookPath = initialBookPath;
   if (isBmpFile(initialBookPath)) {
     onGoToBmpViewer(initialBookPath);