docs: Restructure styling documentation and add theme toggle docs

- Simplify CLAUDE.md from ~460 to ~145 lines with imports
- Add Theme System section to css-design-system.md (3 modes: auto/light/dark)
- Document theme toggle UI, localStorage persistence, CSS priority order
- Add paths: frontmatter to authentication.md and company-period.md
- Update DESIGN_TOKENS.md Dark Mode section with toggle documentation
- Clean auto-build-memory.md header (remove non-existent auto-sync reference)
- Remove non-existent plugin from settings.json

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude/rules/authentication.md

@@ -1,3 +1,7 @@
+---
+paths: "{backend,shared}/**/*.py,src/**/*.{js,vue}"
+---
+
 # Authentication Rules
 
 ## JWT Token Structure (IMMUTABLE)