zen: switch from twilight nightly to stable channel

twilight re-publishes under the same 1.22t tag, so the pinned hash goes
stale and `nh home switch` fails with a fixed-output hash mismatch. Move
to the stable `default` package (branded zen-beta, 1.21.5b), which uses
proper versioned releases.

- zen-browser.nix: twilight-official -> default
- xdg.nix: zen-twilight.desktop -> zen-beta.desktop mimeapp defaults
- flake.lock: bump zen-browser input

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

home/common/programs/zen-browser.nix

@@ -1,4 +1,6 @@
 { inputs, pkgs, ... }:
 {
-  home.packages = [ inputs.zen-browser.packages.${pkgs.stdenv.hostPlatform.system}.twilight-official ];
+  # `default` is Zen's stable channel (branded "zen-beta", binary/desktop `zen-beta`).
+  # Avoids twilight's recurring hash-mismatch breakage (nightly re-tags under the same version).
+  home.packages = [ inputs.zen-browser.packages.${pkgs.stdenv.hostPlatform.system}.default ];
 }