Feature: go to text/start reference in epub guide section at first start (#156)

This parses the guide section in the content.opf for text/start
references and jumps to this on first open of the book.

Currently, this behavior will be repeated in case the reader manually
jumps to Chapter 0 and then re-opens the book. IMO, this is an
acceptable edge case (for which I couldn't see a good fix other than to
drag a "first open" boolean around).

---------

Co-authored-by: Sam Davis <sam@sjd.co>
Co-authored-by: Dave Allie <dave@daveallie.com>

src/activities/reader/EpubReaderActivity.cpp

@@ -65,6 +65,16 @@ void EpubReaderActivity::onEnter() {
     }
     f.close();
   }
+  // We may want a better condition to detect if we are opening for the first time.
+  // This will trigger if the book is re-opened at Chapter 0.
+  if (currentSpineIndex == 0) {
+    int textSpineIndex = epub->getSpineIndexForTextReference();
+    if (textSpineIndex != 0) {
+      currentSpineIndex = textSpineIndex;
+      Serial.printf("[%lu] [ERS] Opened for first time, navigating to text reference at index %d\n", millis(),
+                    textSpineIndex);
+    }
+  }
 
   // Save current epub as last opened epub
   APP_STATE.openEpubPath = epub->getPath();