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.

.claude/adr/0005-stock-firefox-policy-extensions.md

@@ -32,3 +32,6 @@ The policy path gives up build-time reproducibility of the extension binaries, a
 - First launch after a fresh build requires network to fetch the add-ons.
   An offline first boot comes up with the extensions not yet present, populating them once online.
 - Moving to a pinned-package posture later would mean changing the browser variant as well, since the two are coupled — the reason this is captured as a decision rather than a detail.
+- The no-pinned-package rule scopes to the three functional extensions, which is where the signature-enforcement conflict bites.
+  Nord chrome theming comes from the Stylix Firefox Color add-on, a signed add-on that Stylix pins and manages, so it loads on stock mainline Firefox and adds no flake input of ours.
+  That is a bounded, deliberate exception, not a reversal: it is what lets the browser be themed from the shared Stylix scheme without hand-written chrome CSS.

modules/desktop/firefox.nix

@@ -75,9 +75,12 @@ in
       };
 
       # Nord chrome from the shared Stylix scheme, against the one profile.
+      # colorTheme recolours the toolbar and tabs, which the target does not do
+      # on its own, through the Stylix-managed Firefox Color add-on.
       stylix.targets.firefox = {
         enable = true;
         profileNames = [ "default" ];
+        colorTheme.enable = true;
       };
 
       # Links opened from other applications land in Firefox.