feat: implement monochrome design system across entire frontend

Migrate all 84 frontend files from the old themed/colored design to a
monochrome glass-morphism design system. Pure black backgrounds, white
text with opacity levels, glass-card components with backdrop-blur, and
functional color reserved for status indicators only.

Foundation: remap CSS variables to monochrome, simplify Tailwind config,
remove theme toggle, convert brand logo/wordmark to white. Pages: all
14 pages updated. Components: all common, library, session, step-library,
tree-editor, tree-preview, admin, and subscription components converted.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -2,11 +2,9 @@ import { useEffect } from 'react'
 import { RouterProvider } from 'react-router-dom'
 import { router } from '@/router'
 import { useAuthStore } from '@/store/authStore'
-import { useThemeStore } from '@/store/themeStore'
 
 function App() {
   const { isAuthenticated, fetchUser, setLoading } = useAuthStore()
-  const { theme, setTheme } = useThemeStore()
 
   useEffect(() => {
     // On app load, check if we have a token and fetch user data
@@ -20,19 +18,6 @@ function App() {
     }
   }, [])
 
-  // Listen for system theme changes
-  useEffect(() => {
-    const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)')
-    const handleChange = () => {
-      if (theme === 'system') {
-        setTheme('system') // Re-apply to update resolvedTheme
-      }
-    }
-
-    mediaQuery.addEventListener('change', handleChange)
-    return () => mediaQuery.removeEventListener('change', handleChange)
-  }, [theme, setTheme])
-
   return <RouterProvider router={router} />
 }