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/pages/public/ContractResponsePage.tsx

@@ -81,8 +81,9 @@ export const ContractResponsePage: React.FC = () => {
   // Honour branding dark/light mode the same way QuoteResponsePage
   // does — without this the page renders in light regardless of admin
   // settings. The wrapper styling below still has `dark:` variants
-  // so the page reads cleanly in either mode.
-  usePublicDarkMode();
+  // so the page reads cleanly in either mode. `isDark` drives the
+  // theme-aware logo pick in the header.
+  const { isDark } = usePublicDarkMode();
 
   const canvasRef = useRef<HTMLCanvasElement>(null);
   const padRef = useRef<SignaturePad | null>(null);
@@ -233,6 +234,18 @@ export const ContractResponsePage: React.FC = () => {
       <div className="max-w-3xl mx-auto py-8 px-4">
         {/* Issuer header — same shape as QuoteResponsePage. */}
         <div className="text-center mb-6">
+          {(() => {
+            const logo = isDark
+              ? (c.issuer?.logoUrlDark || c.issuer?.logoUrl)
+              : (c.issuer?.logoUrl || c.issuer?.logoUrlDark);
+            return logo ? (
+              <img
+                src={logo}
+                alt={c.issuer?.companyName || 'Logo'}
+                className="mx-auto mb-3 h-16 object-contain"
+              />
+            ) : null;
+          })()}
           {c.issuer?.companyName && (
             <h2 className="text-xl font-bold">{c.issuer.companyName}</h2>
           )}

frontend/src/pages/public/PaymentCheckPage.tsx

@@ -260,12 +260,16 @@ export const PaymentCheckPage: React.FC = () => {
 };
 
 const BrandingHeader: React.FC<{ issuer: PaymentCheckIssuer | null }> = ({ issuer }) => {
-  if (!issuer || (!issuer.logoUrl && !issuer.companyName)) return null;
+  const { isDark } = usePublicDarkMode();
+  if (!issuer || (!issuer.logoUrl && !issuer.logoUrlDark && !issuer.companyName)) return null;
+  const logo = isDark
+    ? (issuer.logoUrlDark || issuer.logoUrl)
+    : (issuer.logoUrl || issuer.logoUrlDark);
   return (
     <header className="text-center mb-8">
-      {issuer.logoUrl && (
+      {logo && (
         <img
-          src={issuer.logoUrl}
+          src={logo}
           alt={issuer.companyName || 'Logo'}
           className="mx-auto h-16 w-auto object-contain mb-3"
         />

frontend/src/pages/public/QuoteResponsePage.tsx

@@ -39,7 +39,8 @@ export const QuoteResponsePage: React.FC = () => {
   // Apply dark mode per the branding settings (forced dark/light)
   // or fall back to the OS preference. Without this the public
   // quote page renders in light mode regardless of admin settings.
-  usePublicDarkMode();
+  // `isDark` drives the theme-aware logo pick below.
+  const { isDark } = usePublicDarkMode();
 
   const { data, isLoading, isError, refetch } = useQuery({
     queryKey: ['public-quote', token],
@@ -153,13 +154,18 @@ export const QuoteResponsePage: React.FC = () => {
             the backend storage directory directly. */}
         {quote.issuer && (
           <div className="text-center mb-6">
-            {quote.issuer.logoUrl && (
-              <img
-                src={quote.issuer.logoUrl}
-                alt={quote.issuer.companyName || 'Logo'}
-                className="mx-auto mb-3 h-16 object-contain"
-              />
-            )}
+            {(() => {
+              const logo = isDark
+                ? (quote.issuer.logoUrlDark || quote.issuer.logoUrl)
+                : (quote.issuer.logoUrl || quote.issuer.logoUrlDark);
+              return logo ? (
+                <img
+                  src={logo}
+                  alt={quote.issuer.companyName || 'Logo'}
+                  className="mx-auto mb-3 h-16 object-contain"
+                />
+              ) : null;
+            })()}
             <h2 className="text-xl font-bold">{quote.issuer.companyName}</h2>
             {quote.issuer.website && (
               <p className="text-sm text-neutral-500 dark:text-neutral-400">{quote.issuer.website}</p>