fix(gallery): leave a visible gap between filter bar and hero header (#624)

When a gallery uses the 'hero' header_style AND the admin enables the
filter bar (search + sort), the search/sort row glued itself to the top
of the hero image. Root cause: HeroHeader carries a decorative `-mt-6`
on its outer div (so it can bleed flush against the page header when
nothing else is above), and that exactly cancelled the wrapper's `mt-6`
between PhotoFilterBar and PhotoGridWithLayouts.

Fix: when the filter bar is shown above a hero header, the grid wrapper
uses `mt-12` instead of `mt-6` so the hero's bleed leaves a 24px net gap
rather than zero. The no-filter-bar case keeps the original flush bleed.

Also tidied up: extract the filter-bar-shown predicate to a named const
so the two reads (conditional render + wrapper class) can't drift apart.

frontend/src/components/gallery/GalleryView.tsx

@@ -697,6 +697,9 @@ export const GalleryView: React.FC<GalleryViewProps> = ({ slug, event }) => {
   const headerStyle = data?.event?.header_style || theme.headerStyle || 'standard';
   const isHeroHeader = headerStyle === 'hero';
   const showSidebar = theme.controlsStyle === 'sidebar';
+  const filterBarShown = !showSidebar
+    && settingsData?.gallery_show_filter_bar !== false
+    && (data?.photos?.length ?? 0) > 0;
 
   // Full-page layouts (gallery-premium, gallery-story) have their own integrated UI
   // Skip all wrapper elements (header, footer, sidebar, filters) for these layouts
@@ -922,7 +925,7 @@ export const GalleryView: React.FC<GalleryViewProps> = ({ slug, event }) => {
             filter bar globally, and when the gallery actually has photos
             (avoids the empty "Search photos by filename" row in the screenshot
             from discussion #317). */}
-        {!showSidebar && settingsData?.gallery_show_filter_bar !== false && (data?.photos?.length ?? 0) > 0 ? (
+        {filterBarShown ? (
           <div className="mt-6">
             <PhotoFilterBar
               categories={data.categories}
@@ -945,8 +948,11 @@ export const GalleryView: React.FC<GalleryViewProps> = ({ slug, event }) => {
         </div>
       ) : null}
 
-        {/* Photo Grid */}
-        <div className={showSidebar ? "mt-6" : "mt-6"}>
+        {/* Photo Grid — when the hero header sits directly under the filter
+            bar, double the wrapper margin (mt-12) so the hero's decorative
+            `-mt-6` bleed leaves a visible gap instead of gluing the filter
+            bar to the hero image (issue #624). */}
+        <div className={filterBarShown && isHeroHeader ? "mt-12" : "mt-6"}>
           <PhotoGridWithLayouts 
             photos={filteredPhotos} 
             slug={slug}