refactor: replace redundant mkForce with plain values and mkDefault

Each of these overrode a value nothing else set at a higher priority:

- qemu-guest: btrbk, hardwareTools and cloud-init have no competing
  definition on the guests.
- mdns: nixpkgs' sunshine module sets avahi.publish.* with mkDefault.
- computer-use: i18n now sets the time zone with mkDefault.
- darwin terminal: the shared ghostty/kitty font sizes are now mkDefault.
- shoukei: sunshine is mkDefault false in remote-desktop.nix, zram's
  memoryPercent is mkDefault 50; supportedFilesystems now merges instead
  of discarding the fsType entries other modules add.
- youko: nothing enables docker.
- nixpak: pasta.enable and sockets.pipewire are mkDefault in the shared
  modules, sockets.pulse already was.

Every NixOS host's toplevel drvPath is unchanged; on fern and frieren the
terminal font sizes and time zone evaluate to the same values.

Assisted-by: Claude Code (claude-opus-5-5)

home/darwin/terminal.nix

@@ -1,8 +1,8 @@
-{ lib, ... }:
+_:
 let
   fontSize = 15;
 in
 {
-  programs.ghostty.settings.font-size = lib.mkForce fontSize;
-  programs.kitty.font.size = lib.mkForce fontSize;
+  programs.ghostty.settings.font-size = fontSize;
+  programs.kitty.font.size = fontSize;
 }