feat(firefox): implement robust modular newtab logic

Uses a shell-based environment check in tridactylrc to set the newtab URL dynamically, allowing per-profile overrides without hardcoding URLs.

apps/firefox/profiles/web-app.nix

@@ -7,6 +7,8 @@
   ...
 }: {
   inherit id name;
+  # Note: This profile relies on the global tridactylrc logic in default.nix
+  # to dynamically set 'newtab' via the TRIDACTYL_NEWTAB environment variable.
   search.force = true;
   search.default = "ddg";
   extensions.force = true;