refactor: Modularize CSS into ITCSS architecture + update documentation

CSS Refactoring:
- Split 2,287-line monolith into 6 focused modules
- New structure: Navigation, Scroll, Buttons, Modals, Zoom, Breakpoints
- Organized by ITCSS layers (Interactive + Responsive)
- 245 lines saved through better organization
- Site verified working at localhost:1999

New CSS Files:
- 04-interactive/_navigation.css (357 lines, 8.2KB)
- 04-interactive/_scroll-behavior.css (200 lines, 5.0KB)
- 04-interactive/_buttons.css (184 lines, 4.7KB)
- 04-interactive/_modals.css (487 lines, 16KB)
- 04-interactive/_zoom-control.css (253 lines, 6.3KB)
- 05-responsive/_breakpoints.css (561 lines, 14KB)

Documentation Updates:
- Added doc/12-CSS-ARCHITECTURE.md (comprehensive CSS guide)
- Updated doc/README.md (new entry + correct numbering)
- Updated tests/README.md (test count 8 → 27)
- Updated tests/TEST-SUMMARY.md (coverage expansion)
- Rewrote tests/mjs/README.md (complete test listing)

Removed:
- static/css/04-interactive/_remaining.css (replaced by modules)

tests/TEST-SUMMARY.md

@@ -286,11 +286,11 @@ When adding tests:
 
 ---
 
-**Last Updated**: 2025-11-18
-**Test Count**: 15 active (0-14) - NO archive, NO legacy tests
-**Coverage**: Complete (UI, keyboard, libraries, i18n, modals, mobile, zoom, hover-sync, hyperscript, skeleton loaders, color themes, button positioning)
+**Last Updated**: 2025-11-20
+**Test Count**: 27 active tests - Comprehensive coverage
+**Coverage**: Complete (UI, keyboard, HTMX, i18n, modals, mobile, zoom, hover-sync, hyperscript, skeleton loaders, color themes, button positioning, PDF modal, icons, dark theme, course icons, references)
 **Status**: SINGLE SOURCE OF TRUTH - Production specification
-**Philosophy**: Zero redundancy - Every test is essential and unique
+**Philosophy**: Each test validates specific functionality and edge cases
 
 ### 12-skeleton-language-transitions.test.mjs
 **Purpose**: Skeleton loader animations during language transitions for ALL 13 curriculum sections