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/SdCardFontSystem.h

@@ -50,3 +50,6 @@ class SdCardFontSystem {
   SdCardFontManager manager_;
   std::atomic<bool> registryDirty_{false};
 };
+
+// Global SD card font system instance (defined in main.cpp).
+extern SdCardFontSystem sdFontSystem;