fix: Hide unusable button hints when viewing empty directory (#1253)

## Summary

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

Increase accuracy of button hints and text description in the file
browser when viewing empty directory.
 
* **What changes are included?**

Adjusted button label hint rendering logic in file browser to hide the
"Open", "Up", and "Down" hints when the they are not available due to an
empty directory.

I also changed the NO_BOOKS_FOUND string to NO_FILES_FOUND and updated
translations. File browser shows more than just books so seeing "No
Books Found" really doesn't make sense.

## Additional Context

Very Simple change, here is what that looks like on my device.

<img width="1318" height="879" alt="Untitled (7)"
src="https://github.com/user-attachments/assets/6416c8c8-795d-41a5-9b9f-28d2c26666a0"
/>

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
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**_

lib/I18n/translations/czech.yaml

@@ -14,7 +14,7 @@ STR_CONTINUE_READING: "Pokračovat ve čtení"
 STR_NO_OPEN_BOOK: "Žádná otevřená kniha"
 STR_START_READING: "Začněte číst níže"
 STR_BOOKS: "Knihy"
-STR_NO_BOOKS_FOUND: "Žádné knihy nenalezeny"
+STR_NO_FILES_FOUND: "Nebyly nalezeny žádné soubory"
 STR_SELECT_CHAPTER: "Vybrat kapitolu"
 STR_NO_CHAPTERS: "Žádné kapitoly"
 STR_END_OF_BOOK: "Konec knihy"