fix: Added missing up/down button labels (#935)

## Summary

**What is the goal of this PR?**

In some places, button labels are omitted intentionally because the
button has no purpose in the activity. I noticed a few obvious cases,
like Home > File Transfer and Settings > System > Language, where the up
and down button labels were missing. This change fixes those and all
similar instances I could find.

---

### 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/browser/OpdsBookBrowserActivity.cpp

@@ -189,7 +189,7 @@ void OpdsBookBrowserActivity::render(Activity::RenderLock&&) {
   if (!entries.empty() && entries[selectorIndex].type == OpdsEntryType::BOOK) {
     confirmLabel = tr(STR_DOWNLOAD);
   }
-  const auto labels = mappedInput.mapLabels(tr(STR_BACK), confirmLabel, "", "");
+  const auto labels = mappedInput.mapLabels(tr(STR_BACK), confirmLabel, tr(STR_DIR_UP), tr(STR_DIR_DOWN));
   GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
 
   if (entries.empty()) {