refactor: consolidate color definitions into single source of truth

- Create frontend/colors.mjs as SSOT for all design colors
- Create scripts/generate-css-vars.mjs to auto-generate CSS variables
- Update tailwind.config.ts to import colors from colors.mjs
- Update frontend/package.json: add 'generate-css-vars' script to build pipeline
- Update start_servers.py: include CSS variable generation before npm build
- All color changes now require only one edit in colors.mjs
- Build process auto-syncs CSS variables and Tailwind colors

Eliminates DRY violation where colors were duplicated in:
  - tailwind.config.ts (Tailwind utility classes)
  - globals.css (CSS custom properties)

Single source of truth approach ensures:
  ✓ No inconsistency between Tailwind and CSS variables
  ✓ One place to update colors for all contexts
  ✓ Automated CSS variable generation on every build

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

frontend/package.json

@@ -3,8 +3,9 @@
   "version": "0.2.0",
   "private": true,
   "scripts": {
-    "dev": "next dev",
-    "build": "next build",
+    "generate-css-vars": "node scripts/generate-css-vars.mjs",
+    "dev": "npm run generate-css-vars && next dev",
+    "build": "npm run generate-css-vars && next build",
     "start": "next start",
     "lint": "next lint",
     "test": "vitest",