fix: improve webapp tab UI with CSS Grid and viewport-filling container

Fixes layout issues on the webhooks admin tab:
- Add min-height to container to fill viewport consistently
- Use CSS Grid to overlay tab panes without jumpiness
- Add smooth htmx fade transitions for content swaps
- Adjust vector sync polling interval from 3s to 10s
- Add .playwright-mcp/ to gitignore for test screenshots

The CSS Grid approach allows tabs to overlay without absolute positioning,
preventing content cutoff while maintaining smooth transitions without
container resizing jumps.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.gitignore

@@ -9,3 +9,4 @@ docker-compose.override.yml
 
 # Generated by pytest used to login users
 .nextcloud_oauth_*.json
+.playwright-mcp/