Multifunctional corner button (easier access to site features & more seeding control for the user)

The button in the corner of the zeronet pages can be multifunctional. Mouse hover over the button shows the menu:

-----------MENU START----------

> ZeroHello
> Site menu
> Delete site
> Favorite site
> Directory open
> ------
> Set site filesharing mode:
> [-] Upload anything & download all _optional_ files
> [x] Upload anything [currently default]
> [-] Upload only text files
> [-] Upload only text files, download no IMAGES and VIDEOS (user have to click an image placeholder in order to download
> image, if he does not like it, he click one more time to delete it and put placeholder again)
> [-] Upload nothing

----------MENU END---------

Benefits of such a menu:
- users easily accessing site options (currently hard to find)
- allow more peace of mind to the user thanks to filesharing mode (more users willing to stay and less negative talk about zeronet being scary place where people can accidentaly seed illegal media)
- slighty reduce number of bad content hosted and allow users easier find option to distribute all site files

If there is possibility for zeronet to control download/upload of images/videos, user should have easily accessible choice not to seed any images from a risky site (as seen on the above menu)

The option "[-] No IMAGES and VIDEOS upload&download" should prompt if user want to delete all existing optional images and videos.

The option "[-] Upload anything & download all _optional_ files" may trigger prompt: "You are going to download and distribute EVERYTHING that was and will be published on this site. Current size of this site data: XY GB. Proceed | Cancel".

**Related issues**:
https://github.com/HelloZeroNet/ZeroNet/issues/492
https://github.com/HelloZeroNet/ZeroNet/issues/2133

Review related issues #492 and #2133, then trace the existing corner button and site-menu entry points. Define the scope for the multifunctional menu, file-sharing controls, prompts, and optional media handling before implementation. Done means the menu exposes the requested site actions and the selected seeding and downloading behavior works as described.

由索引模型根据 Issue 内容生成。