Redesign event detail view: full-screen layout with hero image

Replace card-based event detail view with full-screen gradient layout.
Add hero image with gradient overlay, icon-based meta rows, and
"About" section. Content renders directly on the gradient background
with white text for an app-native feel.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/views/__tests__/EventDetailView.spec.ts

@@ -105,7 +105,7 @@ describe('EventDetailView', () => {
     const wrapper = await mountWithToken()
     await flushPromises()
 
-    const dateField = wrapper.findAll('.detail__value')[0]!
+    const dateField = wrapper.findAll('.detail__meta-text')[0]!
     expect(dateField.text()).toContain('(Europe/Berlin)')
     expect(dateField.text()).toContain('2026')
     wrapper.unmount()