- Remove session-events.svelte.ts (~154 LOC)
- Add demo-events.ts with static sample events
- Simplify events.svelte.ts by removing session logic
- Update AuthGateModal for demo mode messaging
- Show auth gate when users try to create/edit events
- Update banner from "Gast-Modus" to "Demo-Modus"
First-time visitors now see realistic sample events instead of an
empty calendar. Clicking on events or trying to create new ones
prompts for login, providing a better UX than the previous session
storage approach where events would be lost on tab close.