feat: add pdf preview & env

[Ambushfall]

Pull Request Template

Description

• Added option to serve pdf without the need for loading other containers.
• Added ENV Variable for switching off and on

Logic is simple, if you set the ENABLE_PDF env to true, on click of a manga file name it will serve the said .pdf file.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist

• My code follows the style guidelines of this project
• My commits follow the commit standards of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have checked my code and corrected any misspellings