login: always render in dark mode regardless of stored preference

The login page is the operator's entry point — it should look the
same every time, not flip between light and dark depending on what
the previous session left in localStorage.

Adds the `dark` class to the login wrapper div instead of
documentElement, so:
- Skyrise's .dark tokens cascade into all descendants (CSS vars defined
  under .dark apply to the subtree).
- After sign-in and navigation, the user's saved light/dark preference
  takes back over for the rest of the app.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/routes/login.tsx

@@ -24,7 +24,10 @@ export default function LoginRoute() {
 
   return (
     <div
-      className="relative isolate flex min-h-svh items-center justify-center p-4"
+      // Force dark mode on the login page regardless of the operator's
+      // saved theme preference. Scoped to this wrapper (not documentElement)
+      // so navigating away after sign-in restores their preferred mode.
+      className="dark relative isolate flex min-h-svh items-center justify-center p-4"
       style={{ background: "var(--background)" }}
     >
       <LoginForm