Add book upload functionality to the web UI

[dfface]

Description:

It would be very useful to upload books directly through the web interface instead of manually copying files into the library directory. Right now, adding a book appears to require filesystem access, SSH/FTP, USB transfer, or some other external method. This is inconvenient, especially when EPUB Browser is running on a remote server or when using the Kindle-focused page.

A built-in upload feature would make the app much more self-contained and easier to use.

Current behavior:

• Books are discovered from the library directory.
• There is no upload button or form in the web UI.
• kindle-library.html is effectively read-only.
• To add a new book, the user must place the file into the library folder manually.

Proposed behavior:

• Add an Upload button or form to the main library page.
• Support uploading at least .epub files, and ideally .pdf files as well, consistent with what the library page already lists.
• Save uploaded files into the configured library directory.
• Show upload progress and clear success/error messages.
• Automatically refresh the book list after a successful upload.
• If feasible, add an upload entry or link on kindle-library.html, or provide a mobile-friendly upload page that can be opened from another device.
• Handle basic security and validation:
  • optional authentication if configured,
  • allowed file extensions/MIME types,
  • maximum file size limit,
  • protection against path traversal,
  • duplicate filename handling.

Why this matters:

Uploading books from the browser would remove the need for external tools and manual file management. It would make EPUB Browser easier for non-technical users, more convenient for remote setups, and much more practical for Kindle users who want to add books without dealing with the underlying filesystem.

Additional context:

• PDF upload should ideally be accepted alongside EPUB, especially if PDF reading support is added later.
• If PDF support is not ready yet, the upload feature could still accept EPUB first and add PDF support afterwards.
• This would make the library workflow feel complete: upload, browse, and read from the same interface.