Flake: desktop packages + kalendee-server / nixosModules for umbrella

## Goal
Ship a Kalendee flake for [KolektivComputer/nix](https://github.com/KolektivComputer/nix):

- `packages..default` (or `kalendee`) — desktop Linux client when targeting desktop
- `packages..kalendee-server` and/or `nixosModules.kalendee` — backend deploy

Umbrella re-exports clear attrs. Pin `nixpkgs.follows` for the umbrella.

## Related
nix epic — server family + desktop policy (one umbrella, no nix-server branch).

Start by inspecting the repository's current build entry points and any existing flake configuration, then identify how the desktop client and server are produced. Define the umbrella outputs for the desktop package, kalendee-server, and nixosModules.kalendee, and verify that the umbrella pins nixpkgs.follows and re-exports the intended attributes.

Written by the indexing model from the issue text.