Implement Share Links Feature

[dev]

As requested in #288, #441, #687, #732, #979, #1215, #1279, #1551, #1802, #3479, #4611, #4712, #5105, #5650 :

This PR implements a "share links" feature that allows users to create publicly accessible shareable links for books, chapters, and pages without requiring authentication.

Features:

• Create and manage share links for books, chapters, and pages
• Public access via secure token-based URLs (/share/{token})
• Navigation support within shared books and chapters
• Share link management interface in settings
• Activity logging for share link operations
• Secure attachment URLs (token required)

[ssddanbrown]

Thanks for offering this PR @dev.

To be honest though, when looking to implement this myself before I've thought through this style of approach but never really been happy with it, due to the large amount of duplication, re-implementation and the wide general surface-area/impact.

Looking at this PR now, I still feel the same way.
That's not to say there is a better way, just that I'm not sure that the surface area/complexity added by this approach is worthwhile long term.