refactor(wisdom): decentralize module defaults and update conventions

Move Home Manager module defaults from a centralized `desktop-home-defaults.nix`
file into their respective `wisdom` slices. This change aligns with the
updated documentation regarding module ownership, where defaults live
with the option that owns them rather than in a shared defaults file.

Additionally, update `docs/conventions.md` to reflect this new pattern for
defining `mkEnableOption` defaults and move base git/encryption
configurations into the core `wisdom` module.

- Delete `modules/hosts/desktop-home-defaults.nix`
- Update `modules/wisdom/default.nix` to include shared git and package defaults
- Add `default = ...` to `mkEnableOption` in various `wisdom` modules
- Update `docs/conventions.md` with new HM slice guidelines

modules/wisdom/browsers/chromium-hevc.nix

@@ -155,13 +155,15 @@
           `failed Initialize()ing the frame pool` in Jellyfin.
 
           Requires `chiasson.system.chromiumHevc.enable` on NixOS.
-        '';
+        '' // {
+          default = true;
+        };
 
         packages = lib.mkOption {
           type = lib.types.listOf (
             lib.types.enum (lib.attrNames browserCatalog)
           );
-          default = [ "google-chrome" ];
+          default = if pkgs.stdenv.hostPlatform.isAarch64 then [ "chromium" ] else [ "google-chrome" ];
           description = "Chromium-based browsers to wrap.";
         };