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/input.css

@@ -0,0 +1,34 @@
+@import "tailwindcss";
+@config "./tailwind.config.js";
+
+@layer base {
+  @font-face {
+    font-family: 'Inter Variable';
+    src: url('/static/fonts/Inter-Variable.woff2') format('woff2-variations');
+    font-weight: 100 900;
+    font-display: swap;
+  }
+  @font-face {
+    font-family: 'JetBrains Mono Variable';
+    src: url('/static/fonts/JetBrainsMono-Variable.woff2') format('woff2-variations');
+    font-weight: 100 800;
+    font-display: swap;
+  }
+  body {
+    @apply font-sans bg-white text-brand-navy antialiased;
+  }
+  h1, h2, h3 { @apply font-black; letter-spacing: -2.5px; }
+  h1 { letter-spacing: -3px; }
+}
+
+@layer utilities {
+  .grad-text {
+    @apply bg-brand-grad bg-clip-text text-transparent;
+  }
+  .grad-bg {
+    @apply bg-brand-grad text-white;
+  }
+  .eyebrow {
+    @apply text-[11px] uppercase font-bold tracking-[0.18em];
+  }
+}