fix(firefox): silence configPath and pkgs.system deprecation warnings

Rebuild after the lock-maintenance bump surfaced two home-manager
warnings on the Firefox config added in #17:

- pin programs.firefox.configPath = ".mozilla/firefox" (the legacy
  location the system Firefox uses) to silence the stateVersion<26.05
  default-change warning,
- address the add-on by pkgs.stdenv.hostPlatform.system (pkgs.system is
  deprecated).

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

lyrathorpe/home/desktop.nix

@@ -72,13 +72,18 @@
   programs.firefox = {
     enable = true;
     package = null;
+    # Keep the legacy profile location (~/.mozilla/firefox) -- that is where the
+    # system Firefox actually looks; pin it explicitly to silence the
+    # stateVersion<26.05 default-change warning (the new XDG path depends on
+    # Firefox's own profile support).
+    configPath = ".mozilla/firefox";
     profiles.${username} = {
       id = 0;
       isDefault = true;
       extensions = {
         force = true;
         packages = [
-          inputs.firefox-addons.packages.${pkgs.system}.catppuccin-mocha-mauve
+          inputs.firefox-addons.packages.${pkgs.stdenv.hostPlatform.system}.catppuccin-mocha-mauve
         ];
       };
       settings = {