Add 'create new deck' from home page

What

The index page (/) currently lists existing slides in a grid. It should also make it dead simple to start a fresh deck from scratch.

Ideas

• "New Deck" button on the index page that clears existing slides and scaffolds a blank slide-01-title.html
• Or a "Duplicate Deck" option that copies the current slides as a starting point
• Could hit a /api/new-deck endpoint that the server handles (wipe slides, create a starter)
• Bonus: template picker — "blank", "woltspace talk", "project pitch", etc.

Why

This is DAM #001. If someone clones the repo, they should be able to go to / and start their own deck without touching the terminal. That's the whole point of a shareable app — the wolt (or the human) just clicks and goes.

Context

Right now someone has to manually delete slide-*.html files and create new ones. Works, but not very DAM-like.

Review the index page at / and the existing slide-*.html files first; the issue leaves the implementation choice open between a new-deck action, duplication, and a /api/new-deck endpoint. Done means a cloned repo can start a fresh deck from the home page without manually editing or deleting files.