feat(firefox): implement dynamic tridactyl newtab via env var

Allows per-profile newtab overrides using TRIDACTYL_NEWTAB environment variable.

apps/firefox/default.nix

@@ -16,7 +16,7 @@
     ];
 
     workspace = [
-      "special:youtube, on-created-empty:uwsm app -- firefox -P YouTube --name youtube"
+      "special:youtube, on-created-empty:uwsm app -- env TRIDACTYL_NEWTAB=https://youtube.com firefox -P YouTube --name youtube"
     ];
 
     windowrulev2 = [
@@ -38,6 +38,9 @@
     bind J tabnext
     bind K tabprev
     unbind <C-e>
+
+    " Set newtab from environment variable if available (provided by native messenger)
+    js tri.native.run("printenv TRIDACTYL_NEWTAB").then(r => { if (r.stdout.trim()) tri.config.set("newtab", r.stdout.trim()) })
   '';
 
   programs.firefox = {