[feat] token-based theming

### App

Slides

### Problem / motivation

Changing a presentation's theme only affects newly added slides. Existing slides keep their original styling because theme values are copied at slide-creation time.

### Proposed solution

Introduce theme tokens via CSS custom properties. Elements store values like `color: var(--theme-text-body)`, the editor root binds the vars from the theme's palette, and a theme change re-renders everything instantly with no data rewrite. Old decks with literal hex values keep rendering unchanged, so no migration is needed.

Scope: palette JSON field on template Presentations, tokenized Light/Dark fixtures, CSS var binding in the editor, tokenized new-element defaults, and picker UX with theme swatches (custom hex detaches from theme). Existing content is not retro-tokenized.