feat(16-04): add TanStack Router file-based routes and app shell layout

Create route structure with __root, index (redirect to /initiatives),
initiatives/index (dashboard stub), initiatives/$id (detail stub with
type-safe params), and inbox (stub). Add AppLayout with persistent nav
header matching wireframe: title, New Initiative button, navigation tabs
with active highlighting. Disabled tabs for Agents/Tasks/Settings (out
of scope). Replace temporary health-check App with RouterProvider.
Route tree auto-generated by TanStack Router Vite plugin.

packages/web/src/routes/initiatives/$id.tsx

@@ -0,0 +1,16 @@
+import { createFileRoute } from '@tanstack/react-router'
+
+export const Route = createFileRoute('/initiatives/$id')({
+  component: InitiativeDetailPage,
+})
+
+function InitiativeDetailPage() {
+  const { id } = Route.useParams()
+  return (
+    <div>
+      <h1 className="text-2xl font-bold">Initiative Detail</h1>
+      <p className="text-muted-foreground mt-2">Initiative ID: {id}</p>
+      <p className="text-muted-foreground mt-1">Content coming in Phase 18</p>
+    </div>
+  )
+}