feat: Long Click for File Deletion through File Browser (#909)

## Summary

* **What is the goal of this PR?** (e.g., Implements the new feature for
file uploading.)

Allow users to better manage their epub library by offloading unwanted
or finished books and other files. Resolves #893

* **What changes are included?**

Added Delete Book shortcut in the fil browser. Delete function
implements the new ConfirmationActivity to show file name and solicit
user interaction before either returning to the file browser on a press
of the back button, or proceeding to delete. Delete function then
deletes the file and returns user to the file browser menu at the
current directory. Video of it working on my machine attached here:


https://github.com/user-attachments/assets/329b0198-9e97-45ad-82aa-c39894351667


## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).

Certainly potential risks associated with file deletion. Please let me
know if there are any concerns that need to be better addressed. I think
this is a very good feature to have to go along with the new screenshots
so you don't get stuck with a bunch of extra files on your device. Also
I did add this to the user guide.

---

### 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? _**YES**_

---------

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>
Co-authored-by: Егор Мартынов <martynovegorOF@yandex.ru>
Co-authored-by: Arthur Tazhitdinov <lisnake@gmail.com>
Co-authored-by: Zach Nelson <zach@zdnelson.com>

USER_GUIDE.md

@@ -84,7 +84,8 @@ See [Reading Mode](#4-reading-mode) below for more information.
 The Browse Files screen acts as a file and folder browser.
 
 * **Navigate List:** Use **Left** (or **Volume Up**), or **Right** (or **Volume Down**) to move the selection cursor up and down through folders and books. You can also long-press these buttons to scroll a full page up or down.
-* **Open Selection:** Press **Confirm** to open a folder or read a selected book.
+* **Open Selection:** Press **Confirm** to open a folder or read a selected book. 
+* **Delete Files:** Hold and release **Confirm** to delete the selected file. You will be given an option to either confirm or cancel deletion. Folder deletion is not supported.
 
 ### 3.4 Recent Books Screen