Migrate styled-jsx to CSS Modules

[tmshv]

Problems

6 components use <style jsx> which requires 'use client' in Next.js 15 and is inconsistent with the rest of the codebase (CSS Modules). Migrate all to CSS Modules.

Files to migrate

• src/components/UnitHighlight/index.tsx
• src/components/Button/index.tsx
• src/components/Z1/index.tsx
• src/components/UnitCanvas/index.tsx
• src/special/heterotopia/Highlight.tsx
• src/special/heterotopia/Title.tsx

Additional: rewrite UnitCanvas as functional component

UnitCanvas is the only class component in the project. While migrating its styles, also:

• Rewrite as functional component with useRef/useEffect
• Add null guards for canvas and context (currently uses non-null assertions)
• Clear stopTimeout in cleanup to fix timer leak
• Add 'use client' directive