perf(chatbot): lazy-load via next/dynamic + convert (marketing) layout to RSC

[abdout]

Severity

P1 — Performance

Context

`src/app/[lang]/(marketing)/layout.tsx` was `"use client"` because it held a `useRef` for the chatbot's imperative `openChat()` method. Investigation showed the `onChatClick` prop was drilled through 4 layers (`layout → SiteHeader → SiteHeaderClient → RightActions`) but never actually invoked — `RightActions` accepts the prop and ignores it. `MobileHeader` doesn't even receive it. So the entire prop chain was dead plumbing keeping the layout client-side for nothing.

Fix

• Drop dead `onChatClick` plumbing across 4 files
• Convert `(marketing)/layout.tsx` from client to server component
• Lazy-load `ChatbotContent` via `next/dynamic` (`ssr: false`) inside a tiny `` client island

Impact

• All marketing pages (`/`, `/pricing`, `/service`) become RSC by default again — chatbot bundle no longer in the marketing critical path
• Chatbot widget is fetched only when needed; reduces initial JS payload and TTI
• Honors `PERFORMANCE.md`'s claim of "lazy loading of chat window" which was actually unimplemented

Acceptance criteria

• `(marketing)/layout.tsx` has no `"use client"` directive
• Network tab shows chatbot chunk loading after initial page render, not in critical path
• Chatbot still opens via its in-page button on `/` and `/pricing`
• No `onChatClick` references anywhere in src/

Reference

Plan: PR-13.