[Feature Request]: Shared vaults/folders with per-user permissions

[crispyduck00]

Confirmation Checklist / 确认检查清单

• I have read and confirmed the checklist above. / 我已阅读以上清单并确认。

Background & Motivation / 背景与动机

Background

I would like to use FNS with separate accounts for family members while keeping
personal notes private.

Currently, sharing appears to create read-only public links. Collaborative editing
seems to require sharing the same FNS account/token and the entire vault.

Desired Feature / 想要实现或优化的功能

Desired feature

Allow a vault owner to invite other FNS users to:

• an entire vault, or ideally a selected folder;
• with read-only or read/write permission;
• using each user's own account and token;
• with the same permissions applied to REST and MCP access.

Example:

• Andreas has a private personal vault.
• Conny has her own private vault.
• Both can edit Family/ or a separate Family vault.
• Each person keeps their own login.
• An AI token can be restricted to the shared area.

It would also be useful if revision history identified which user or client made
a change.

Relation to existing request

Issue #339 requests folder-level permissions for tokens. This request extends that
idea to user-to-user sharing and collaborative vault/folder membership.

Questions

• Is shared multi-user editing already possible without sharing credentials?
• Are shared vaults or folder ACLs planned?
• Would simultaneous edits still use the existing merge mechanism, or is stronger
  conflict handling/CRDT collaboration planned?

Final Step / 最后一步

• I believe the above description is detailed enough for the developer to understand the feature I want. If my issue is not filled out as requested, it may be closed without explanation.
  我认为上述的描述已经足以详细，以便插件开发人员理解您想要的功能。如果我的 Issue 没有按照上述的要求填写，可能会被 无条件关闭。