fix(branding): unify hero logo SIZE the same way as visibility (#756)

Follow-on to the visibility fix in this PR — hero_logo_size had the same
split-brain: GalleryLayout read the global branding_logo_size live while
the hero-header path used the per-event snapshot, so a hero logo could
render at different sizes on different layouts and the global size didn't
reach hero-header galleries.

Now mirrored on the visibility model: NULL per-event hero_logo_size =
inherit branding_logo_size; explicit = override.

- Migration 153: hero_logo_size nullable + backfill NULL so existing
  galleries inherit the global size (restores GalleryLayout's prior
  live-global behaviour and fixes the hero-header staleness).
- Creation stores NULL unless explicit; gallery.js resolves
  per-event ?? global and sends the effective size.
- GalleryLayout now consumes that resolved size for the hero logo (new
  heroLogoSize prop) instead of the global — both render paths match.
- Admin size control gains a 'Use branding default' (inherit) option.

Verified: migration on SQLite + PG; live resolution (inherit follows
global both ways, override wins); creation stores NULL on PG; tsc clean,
106 adminEvents+gallery tests pass, build green.

frontend/src/pages/admin/EventDetailsPage.tsx

@@ -309,7 +309,8 @@ export const EventDetailsPage: React.FC = () => {
       // Load hero logo settings from event. Preserve null = "inherit global"
       // (#756) — don't collapse it to true, or saving would snapshot an override.
       hero_logo_visible: event.hero_logo_visible ?? null,
-      hero_logo_size: event.hero_logo_size || 'medium',
+      // Preserve null = "inherit global size" (#756) — don't collapse to medium.
+      hero_logo_size: event.hero_logo_size ?? null,
       hero_logo_position: event.hero_logo_position || 'top',
       // Hero image anchor position (#162)
       hero_image_anchor: event.hero_image_anchor || 'center',

frontend/src/pages/admin/event-details/EventInformationCard.tsx

@@ -620,10 +620,12 @@ export const EventInformationCard: React.FC<EventInformationCardProps> = ({
                       {t('events.heroLogoSize', 'Logo Size')}
                     </label>
                     <select
-                      value={editForm.hero_logo_size}
-                      onChange={(e) => setEditForm(prev => ({ ...prev, hero_logo_size: e.target.value as 'small' | 'medium' | 'large' | 'xlarge' }))}
+                      // '' = inherit the global branding logo size (#756).
+                      value={editForm.hero_logo_size ?? ''}
+                      onChange={(e) => setEditForm(prev => ({ ...prev, hero_logo_size: e.target.value === '' ? null : e.target.value as 'small' | 'medium' | 'large' | 'xlarge' }))}
                       className="w-full sm:w-48 px-3 py-2 border border-neutral-300 dark:border-neutral-600 bg-white dark:bg-neutral-800 text-neutral-900 dark:text-neutral-100 rounded-md shadow-sm focus:ring-primary-500 focus:border-accent-dark text-sm"
                     >
+                      <option value="">{t('events.heroLogoInherit', 'Use branding default')}</option>
                       <option value="small">{t('events.heroLogoSizeSmall', 'Small')}</option>
                       <option value="medium">{t('events.heroLogoSizeMedium', 'Medium')}</option>
                       <option value="large">{t('events.heroLogoSizeLarge', 'Large')}</option>

frontend/src/pages/admin/event-details/types.ts

@@ -26,7 +26,7 @@ export type EditFormState = {
   use_canvas_rendering: boolean;
   // Hero logo settings. null = inherit the global branding toggle (#756).
   hero_logo_visible: boolean | null;
-  hero_logo_size: 'small' | 'medium' | 'large' | 'xlarge';
+  hero_logo_size: 'small' | 'medium' | 'large' | 'xlarge' | null;
   hero_logo_position: 'top' | 'center' | 'bottom';
   // Hero image anchor position (#162) – keyword or "X% Y%" focal point
   hero_image_anchor: string;
@@ -74,7 +74,7 @@ export const INITIAL_EDIT_FORM: EditFormState = {
   use_canvas_rendering: false,
   // Hero logo settings — null = inherit global branding toggle (#756)
   hero_logo_visible: null,
-  hero_logo_size: 'medium',
+  hero_logo_size: null,
   hero_logo_position: 'top',
   // Hero image anchor position (#162)
   hero_image_anchor: 'center',