support magic folder ids in the remaining endpoints

[shepilov]

Follow-up to #4921.

Use Instance.ResolveDirID wherever an API accepts a directory ID. The first changes cover file/folder creation, metadata, contents, and size under /files.

What's left

• Copy: resolve the DirID query parameter.
• PATCH and batch PATCH: resolve source IDs and parent IDs (dir_id or the parent relationship).
• Other folder routes: HEAD, trash/delete, restore, and reference/sync settings. Decide how aliases should work for trash and restore; don't restore old contents just to resolve an ID.
• Archives and bulk requests: resolve directory IDs before checking permissions or saving the request.
• Notes: resolve an explicit dir_id in POST /notes. The default Notes folder already uses the resolver.
• Shared drives: resolve path IDs, copy destinations, PATCH parents, and share/permission folder targets on the owner's instance, before access checks. Keep encoded slashes intact when proxying. Test both local and remote owners.
• Moves between drives: resolve source.dir_id and dest.dir_id on their respective instances.

Keep the same rules: authenticate first, allow missing magic folders to be created, then check permissions for the actual operation. Store real directory IDs. Leave normal IDs and application references unchanged.

Reuse the resolver; don't add folder creation to VFS getters. Add tests and update the docs as each endpoint is wired up.