feat: manage Handy preferences and startup declaratively

docs/constellation.md

@@ -29,8 +29,8 @@ use a flake to declare its tools without Nix owning the surrounding workspace.
   interaction with useful connections to the rest of the environment.
 - Use Handy for local plain dictation, mainly English with occasional Dutch.
   Use native Wayland automatic paste and optional live text with a streaming model.
-  Niri owns recording shortcuts;
-  model downloads and microphone preferences remain application-owned. Cleanup
+  Nix owns Handy preferences and desktop integration; the user service restores
+  settings on each start. Model downloads and recordings remain application data. Cleanup
   through Codex, a cloud API or a local language model is deferred.
 - Outlook is acceptable as an application, but its interaction and integration
   do not fit as naturally. Accessible mail for tools and agents matters separately