feat(floating-ai): step 4 — build FloatingChat component

Create the floating AI chat popup rendered via portal to document.body.
Uses useAIChat for chat logic, useFloatingChat for position/state,
Framer Motion for enter/exit animations, and pointer-event dragging.

Includes: close on Escape, close on route change, auto-scroll,
auto-focus, window resize clamping, and compact message rendering.

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

src/renderer/components/layout/AppShell.tsx

@@ -56,6 +56,7 @@ import {
 import { Input } from '@/components/ui/input';
 import { Button } from '@/components/ui/button';
 import { AIChatPanel } from '@/components/ai/AIChatPanel';
+import { FloatingChatPortal } from '@/components/ai/FloatingChat';
 import { useTheme } from '@/components/theme-provider';
 import { FloatingChatProvider } from '@/context/FloatingChatContext';
 
@@ -267,6 +268,9 @@ function AppShellInner({ children }: AppShellProps) {
       </SidebarInset>
     </SidebarProvider>
 
+    {/* Floating AI Chat — portal to document.body */}
+    <FloatingChatPortal />
+
     {/* AI Token Dialog — rendered outside Sidebar to avoid layout conflicts */}
     <Dialog open={tokenDialogOpen} onOpenChange={(open) => {
       setTokenDialogOpen(open);