feat(obsidian): install Obsidian as the desktop note-taking vault

Add a standalone modules.desktop.obsidian module and enable it on
neogaia. Give it a waybar window-rewrite icon so its windows show a
recognisable glyph on the workspace indicator.

modules/desktop/obsidian.nix

@@ -0,0 +1,18 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# Obsidian as the desktop note-taking vault.
+let
+  cfg = config.modules.desktop.obsidian;
+  user = config.user.name;
+in
+{
+  options.modules.desktop.obsidian.enable = lib.mkEnableOption "Obsidian as the desktop note-taking vault";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user}.home.packages = [ pkgs.obsidian ];
+  };
+}