M3: seat configurator UI

A "AI seats" page (shadcn, on the design language): manage BYOK model connections (add +
test; the key is write-only), create seats on a team, and configure an agent per seat — name,
the color-graded autonomy dial (draft slate / gated indigo / auto teal), a model connection,
skill toggles from the registry, and docs. Navigable AppShell sidebar (Board / AI seats).

Verified: client `npm run build` clean (1890 modules, tsc + vite).

client/src/App.tsx

@@ -2,6 +2,7 @@ import { Navigate, Route, Routes } from 'react-router'
 import { Toaster } from '@/components/ui/sonner'
 import { BoardPage } from '@/pages/BoardPage'
 import { LoginPage } from '@/pages/LoginPage'
+import { SeatsPage } from '@/pages/SeatsPage'
 import { useAuth } from '@/store/auth'
 
 export default function App() {
@@ -12,6 +13,7 @@ export default function App() {
       <Routes>
         <Route path="/login" element={token ? <Navigate to="/" replace /> : <LoginPage />} />
         <Route path="/" element={token ? <BoardPage /> : <Navigate to="/login" replace />} />
+        <Route path="/seats" element={token ? <SeatsPage /> : <Navigate to="/login" replace />} />
         <Route path="*" element={<Navigate to="/" replace />} />
       </Routes>
       <Toaster richColors position="top-right" />