fix(branding): theme-aware logo on customer-facing public pages

The public quote, contract-signing, and payment-check pages baked a single
light logo (the contract page showed none), so the dark page rendered a
dark-text logo on a dark background.

- usePublicDarkMode now returns { isDark } (reactive) alongside applying
  the .dark class, so pages can pick a theme-aware asset.
- The three public routes now surface both branding logo URLs (logoUrl +
  logoUrlDark) in the issuer block; the contract issuer gains a logo too.
- QuoteResponsePage, ContractResponsePage, and the payment-check
  BrandingHeader pick the dark variant when isDark, falling back to
  whichever exists. Covers the accept/accepted states of each page.

frontend/src/hooks/usePublicDarkMode.ts

@@ -14,16 +14,28 @@
  * Shared by QuoteResponsePage + PaymentCheckPage. Adding a third
  * public-page consumer? Reuse this hook.
  */
-import { useEffect } from 'react';
+import { useEffect, useState } from 'react';
 import { usePublicSettings } from './usePublicSettings';
 
-export function usePublicDarkMode() {
+/**
+ * Returns `{ isDark }` (reactive) in addition to applying the `.dark`
+ * class, so callers can pick a theme-aware asset (e.g. the dark-mode
+ * logo) without re-deriving the mode themselves.
+ */
+export function usePublicDarkMode(): { isDark: boolean } {
   const { data: publicSettings } = usePublicSettings();
+  const forced = publicSettings?.branding_force_color_mode;
+  const [isDark, setIsDark] = useState<boolean>(() => {
+    if (forced === 'dark') return true;
+    if (forced === 'light') return false;
+    return typeof window !== 'undefined'
+      && window.matchMedia('(prefers-color-scheme: dark)').matches;
+  });
   useEffect(() => {
     const root = document.documentElement;
-    const forced = publicSettings?.branding_force_color_mode;
-    const apply = (isDark: boolean) => {
-      if (isDark) root.classList.add('dark');
+    const apply = (dark: boolean) => {
+      setIsDark(dark);
+      if (dark) root.classList.add('dark');
       else root.classList.remove('dark');
     };
     if (forced === 'dark') {
@@ -39,5 +51,6 @@ export function usePublicDarkMode() {
     const listener = (e: MediaQueryListEvent) => apply(e.matches);
     mql.addEventListener('change', listener);
     return () => mql.removeEventListener('change', listener);
-  }, [publicSettings?.branding_force_color_mode]);
+  }, [forced]);
+  return { isDark };
 }