Event draft preview page is left-aligned instead of centered

**What happens:** When you reach the "Event Preview" step of the event creation flow (the screen with the breadcrumb `Organization / Group`, the event title, Details / Date & Time / Online / Capacity sections, and the **Back to Edit** / **Publish** buttons), the entire content block is pinned to the left side of the viewport. The right \~70% of the page is empty whitespace. **Expected:** The preview content should be centered horizontally — either constrained to a max-width container that's centered (matching the rest of the app's page layout), or at minimum follow the same gutter pattern used on the Home / Explore / Organizations pages. **Where:** the **EVENT PREVIEW** screen reached after filling out event details and hitting "Preview" (immediately before "Publish"). The header and footer of the page are correctly full-width — only the preview content body is misaligned. **Repro:** 1. Start creating a new event under any organization. 2. Fill out the required fields (title, date, time, online URL). 3. Click "Preview". 4. Observe the content block is stuck to the left edge of the screen instead of centered. **Notes:** the rest of the app (Home, Explore, Organizations, event detail pages) appears correctly centered, so this is likely a missing `max-w-*` / `mx-auto` (or equivalent container) on the preview-step layout only.