feat(firefox): Nord-theme the chrome via Stylix colorTheme (task 0032)

The base Stylix Firefox target themes only fonts and the reader view, leaving
the toolbar and tabs at Firefox's default look. Enable colorTheme so Stylix
recolours the chrome Nord from the shared scheme through its managed Firefox
Color add-on, with no hand-written chrome CSS and no flake input of ours.

Scope ADR 0005's no-pinned-add-on rule to the three functional extensions and
record the signed, Stylix-managed theming add-on as a deliberate exception.

modules/desktop/firefox.nix

@@ -47,6 +47,10 @@ in
         profiles.default = {
           isDefault = true;
 
+          # The Nord chrome theme is a declared extension setting, so home-manager
+          # owns the extension-settings store, overwriting runtime changes to it.
+          extensions.force = true;
+
           settings = {
             # Sponsored surfaces the policies above do not reach.
             "browser.urlbar.suggest.quicksuggest.sponsored" = false;