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/tailwind.config.js

@@ -1,6 +1,5 @@
 /** @type {import('tailwindcss').Config} */
 export default {
-  darkMode: ["class"],
   content: [
     "./index.html",
     "./src/**/*.{js,ts,jsx,tsx}",
@@ -8,25 +7,7 @@ export default {
   theme: {
     extend: {
       colors: {
-        // ResolutionFlow Brand Colors
-        brand: {
-          gradient: {
-            from: '#818cf8',
-            to: '#a78bfa',
-          },
-          dark: {
-            DEFAULT: '#09090b',
-            card: '#18181b',
-            surface: '#12121c',
-          },
-          text: {
-            primary: '#ffffff',
-            secondary: '#a1a1aa',
-            muted: '#52525b',
-          },
-          border: '#27272a',
-        },
-        // shadcn/ui color system
+        // shadcn/ui color system (monochrome)
         border: "hsl(var(--border))",
         input: "hsl(var(--input))",
         ring: "hsl(var(--ring))",
@@ -68,12 +49,6 @@ export default {
       },
       fontFamily: {
         sans: ['Inter', 'system-ui', '-apple-system', 'BlinkMacSystemFont', 'Segoe UI', 'Roboto', 'sans-serif'],
-        heading: ['Plus Jakarta Sans', 'system-ui', 'sans-serif'],
-        label: ['Outfit', 'system-ui', 'sans-serif'],
-      },
-      backgroundImage: {
-        'gradient-brand': 'linear-gradient(90deg, #818cf8 0%, #a78bfa 100%)',
-        'gradient-brand-hover': 'linear-gradient(90deg, #6366f1 0%, #9333ea 100%)',
       },
     },
   },