[Feature Request] Allow anonymous uploads to shares.

Please consider implementing writable shares where anonymous users can upload files, but not overwrite/delete existing files.

The use case is sharing files with a trustworthy group, e.g. you went on holiday together and everybody uploads the pictures they have taken. Everybody should be able to download all files and also add new ones, but only the owner of the share should be able to delete or overwrite stuff. This way, the link can also be shared with relatives or friends without someone accidentally deleting files.

The issue names no files, tests, or entry points. Start by tracing how shares handle anonymous uploads and permissions, then identify the relevant server and web-app components. Done means anonymous users can add and download files without overwriting or deleting existing files, while share owners retain those controls.

Written by the indexing model from the issue text.