feat: add playground theme switching

[trsoliu]

Summary

• Adds a mini-program playground theme state helper with Default, Learning, and Warm token themes.
• Adds a reusable Taro-safe theme switcher and wires it into the home page, category panel, and demo page shell.
• Keeps the demo catalog and registry coverage test from current main; demo coverage work was already converged by d17f112.

Test Plan

• pnpm --filter @vekui/miniprogram typecheck
• pnpm check:ui
• pnpm build:miniprogram
• pnpm typecheck
• pnpm test
• pnpm registry:build
• pnpm build:docs