docs: revise UI/UX plan — todos showcase, shared CSS, CSP compliance (#54)

Rewrite the UI/UX improvement plan to focus on:
- Todos as the flagship README showcase with two-tab demo GIF
- Shared semantic CSS in livetemplate.css (compact, visually-hidden, inline, chat styles)
- Strict CSP compliance — fix all inline JS/styles across examples
- Leverage latest client features (v0.8.19): lvt-on:change, lvt-el:, lvt-fx:, data-lvt-target
- New CLAUDE.md constraints for future examples

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: adnaan