Fixes #13: CSS performance optimizations

- Add CSS minification support to build script (--minify flag)
  - Pure JS minifier: removes comments, whitespace, redundant chars
  - Reports size reduction percentage
  - Preserves CSS custom properties, data URIs, strings

- Refactor _08_themes.css: extend CSS custom properties pattern
  - Define theme-specific properties per theme (--rpg-content-bg,
    --rpg-content-box-shadow, --rpg-header-text-shadow, etc.)
  - Replace triple-theme selector duplication with single [data-theme]
    selectors — children inherit variables automatically
  - Reduces themes.css from 379 to ~220 lines
  - Reduces style.css from 305KB to 294KB (3.8% reduction)

- Add build:css:min npm script for production builds
- Add 9 tests for CSS build: minification, size reduction, CSS
  variable preservation, data URI preservation, theme refactoring

package.json

@@ -6,6 +6,7 @@
   "type": "module",
   "scripts": {
     "build:css": "node scripts/build-css.js",
+    "build:css:min": "node scripts/build-css.js --minify",
     "test": "jest",
     "validate_locale": "node src/i18n/validator.js --watch",
     "validate_locale_once": "node src/i18n/validator.js"
@@ -18,6 +19,7 @@
     "@babel/preset-env": "^8.0.2",
     "@jest/globals": "^30.4.1",
     "chokidar": "^5.0.0",
+    "execa": "^9.6.1",
     "fs-extra": "^11.3.3",
     "glob": "^13.0.6",
     "jest": "^30.4.2"