Fix NIP-46 QR code styling to use tokenized colors and utility classes

Updated `js/ui/loginModalController.js` to resolve QR code colors from CSS variables (`--color-white`, `--color-black`) instead of hardcoding hex values, ensuring compatibility with the design system.
Updated `components/login-modal.html` to use Tailwind utility classes (`bg-white`, `text-black`, `p-6`) instead of inline styles, improving maintainability and readability.
This ensures the QR code remains scannable (high contrast) while respecting the codebase's tokenized styling approach.

components/login-modal.html

@@ -348,8 +348,7 @@
           <div data-nip46-handshake-panel class="space-y-6">
             <div class="flex flex-col items-center gap-6">
               <div
-                class="flex aspect-square w-full max-w-xs items-center justify-center rounded-2xl border border-border-strong bg-white p-6 shadow-sm"
-                style="background-color: white; color: black;"
+                class="flex aspect-square w-full max-w-xs items-center justify-center rounded-2xl border border-border-strong bg-white text-black p-6 shadow-sm"
                 data-nip46-qr
                 aria-hidden="true"
               >