fix: theme-preset match loop ignores extra fields like logoUrl (#323)

The "which preset does this saved theme match?" loop in BrandingPage and
CreateEventPage was doing a full JSON.stringify equality on preset.config
vs the loaded theme. The previous #323 logo-preservation work means the
saved theme legitimately carries a `logoUrl` (and any other fields the
parent maintains), so the equality check would never match and the
preset summary fell back to "Custom Theme" / Classic Grid even when the
saved theme was structurally Dark Modern, etc.

Compare only on the preset's own keys instead. Surfaced by the new
smoke spec 07-branding-default-on-create-event which would otherwise
pass green against the broken state.

frontend/src/pages/admin/BrandingPage.tsx

@@ -106,9 +106,16 @@ export const BrandingPage: React.FC = () => {
           setBrandingSettings(prev => ({ ...prev, logo_url: formatted.logoUrl }));
         }
 
-        // Try to identify which preset this matches
+        // Try to identify which preset this matches. Compare only on the
+        // fields the preset itself defines so saved themes carrying extras
+        // like a `logoUrl` (preserved through preset changes — see
+        // handlePresetChange) still match the original preset shape.
         for (const [key, preset] of Object.entries(GALLERY_THEME_PRESETS)) {
-          if (JSON.stringify(preset.config) === JSON.stringify(formatted)) {
+          const keys = Object.keys(preset.config);
+          const matches = keys.every((k) =>
+            JSON.stringify((preset.config as any)[k]) === JSON.stringify((formatted as any)[k])
+          );
+          if (matches) {
             setCurrentThemeName(key);
             break;
           }

frontend/src/pages/admin/CreateEventPage.tsx

@@ -213,11 +213,16 @@ export const CreateEventPage: React.FC = () => {
     if (!brandingTheme || Object.keys(brandingTheme).length === 0) return;
     brandingThemeApplied.current = true;
 
-    // Identify which preset (if any) the Branding theme matches, so the
-    // "Theme & Style" panel shows the right name.
+    // Identify which preset (if any) the Branding theme matches. Compare
+    // only on the preset's own fields so saved themes carrying extras
+    // (e.g. logoUrl preserved through preset changes) still match.
     let matchedPreset = 'custom';
     for (const [key, preset] of Object.entries(GALLERY_THEME_PRESETS)) {
-      if (JSON.stringify(preset.config) === JSON.stringify(brandingTheme)) {
+      const keys = Object.keys(preset.config);
+      const matches = keys.every((k) =>
+        JSON.stringify((preset.config as any)[k]) === JSON.stringify((brandingTheme as any)[k])
+      );
+      if (matches) {
         matchedPreset = key;
         break;
       }