feat(marketing): bootstrap Tailwind v4 + design tokens FlexiHub

Adds Tailwind v4 / PostCSS pipeline that compiles to static/css/marketing.css,
to be loaded only by future templates/marketing/** templates and to coexist
with the existing legacy v3 JIT runtime used by index.html / account.html /
admin.html. The legacy v3 runtime stays untouched.

- package.json: postcss-cli build:css and watch:css scripts
- postcss.config.js: @tailwindcss/postcss + autoprefixer
- static/css/tailwind.config.js: brand tokens (b1/b2/b3, navy, navy2, navy3,
  bg, border), font families, brand-grad, cta shadows, FlexiHub keyframes
  (tc-fade-in-up/right, tc-float-y, tc-pulse-glow, plus-breathe). content
  paths scoped to marketing/legal/billing/macros only - purge cannot touch
  legacy templates.
- static/css/input.css: @import "tailwindcss"; + @config directive (Tailwind
  v4 backward-compat for v3-style JS config). @font-face Inter Variable +
  JetBrains Mono Variable (woff2). base layer body font/color, h1-h3
  letter-spacing. utilities: grad-text, grad-bg, eyebrow.
- Dockerfile: new stage 3 'assets-builder' (node:20-alpine) compiles CSS;
  runtime stage copies the built file in via --from=assets-builder, after
  COPY . . so the freshly-built file always wins.
- .gitignore + .dockerignore: exclude node_modules.

Build verified locally: marketing.css = 121 KB minified (Tailwind v4 ships
all default theme tokens + reset properties even with empty content;
realistic baseline, will not grow much as marketing markup is added).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

static/css/tailwind.config.js

@@ -0,0 +1,65 @@
+/** @type {import('tailwindcss').Config} */
+module.exports = {
+  content: ['./templates/marketing/**/*.html', './templates/legal/**/*.html', './templates/billing/**/*.html', './templates/macros/**/*.html', './src/marketing/**/*.py', './src/legal/**/*.py', './src/billing/**/*.py'],
+  darkMode: 'class',
+  theme: {
+    extend: {
+      colors: {
+        brand: {
+          b1: '#0062ff',
+          b2: '#00bdd8',
+          b3: '#00c896',
+          navy: '#060d1a',
+          navy2: '#0b1525',
+          navy3: '#0f1e35',
+          bg: '#f7f9fc',
+          border: '#e6ebf2',
+        },
+      },
+      fontFamily: {
+        sans: ['Inter Variable', 'Inter', 'system-ui', 'sans-serif'],
+        mono: ['JetBrains Mono Variable', 'JetBrains Mono', 'monospace'],
+      },
+      backgroundImage: {
+        'brand-grad': 'linear-gradient(118deg, #0062ff, #00bdd8 52%, #00c896)',
+      },
+      boxShadow: {
+        'cta': '0 4px 20px rgba(0, 98, 255, 0.28)',
+        'cta-hover': '0 8px 32px rgba(0, 98, 255, 0.42)',
+      },
+      borderRadius: {
+        DEFAULT: '0.75rem',
+      },
+      keyframes: {
+        'tc-fade-in-up': {
+          '0%': { opacity: '0', transform: 'translateY(16px)' },
+          '100%': { opacity: '1', transform: 'translateY(0)' },
+        },
+        'tc-fade-in-right': {
+          '0%': { opacity: '0', transform: 'translateX(-16px)' },
+          '100%': { opacity: '1', transform: 'translateX(0)' },
+        },
+        'tc-float-y': {
+          '0%, 100%': { transform: 'translateY(0)' },
+          '50%': { transform: 'translateY(-8px)' },
+        },
+        'tc-pulse-glow': {
+          '0%, 100%': { boxShadow: '0 4px 20px rgba(0, 98, 255, 0.28)' },
+          '50%': { boxShadow: '0 8px 32px rgba(0, 98, 255, 0.42)' },
+        },
+        'plus-breathe': {
+          '0%, 100%': { transform: 'scale(1)' },
+          '50%': { transform: 'scale(1.05)' },
+        },
+      },
+      animation: {
+        'tc-fade-in-up': 'tc-fade-in-up 600ms ease-out forwards',
+        'tc-fade-in-right': 'tc-fade-in-right 600ms ease-out forwards',
+        'tc-float-y': 'tc-float-y 4s ease-in-out infinite',
+        'tc-pulse-glow': 'tc-pulse-glow 3s ease-in-out infinite',
+        'plus-breathe': 'plus-breathe 2s ease-in-out infinite',
+      },
+    },
+  },
+  plugins: [],
+}