feat(web): P1 Vite skeleton beside Next — entry, router, guards, vitest 3

First increment of the approved Phase P RFC (webui-mission). Adds a Vite + React
Router SPA scaffold coexisting with the Next app: index.html with the theme
anti-flash script, src/main.tsx entry, the v1 parity route table under Guest/Auth
guard shells, and a dev proxy (/api, /socket.io ws) to the gateway on 14242 so the
SPA is same-origin in dev. vitest bumped to v3 (vite 8 pairing); existing specs
pass unchanged. Next remains the served app until the P5 cutover.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01ESFAnh2t9HmLwng8oW95St

apps/web/src/spa/guards.tsx

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+import type { ReactElement } from 'react';
+import { Outlet } from 'react-router-dom';
+
+// P1 shells: session-aware redirects arrive with the reworked auth client in P2.
+
+export function GuestGuard(): ReactElement {
+  return <Outlet />;
+}
+
+export function AuthGuard(): ReactElement {
+  return <Outlet />;
+}

apps/web/src/spa/placeholder.tsx

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+import type { ReactElement } from 'react';
+
+export function Placeholder({ title }: { title: string }): ReactElement {
+  return (
+    <main className="flex min-h-screen items-center justify-center">
+      <h1 className="text-xl font-medium">{title}</h1>
+    </main>
+  );
+}

apps/web/src/spa/routes.spec.tsx

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+import { describe, expect, it } from 'vitest';
+import type { RouteObject } from 'react-router-dom';
+import { routes } from '@/routes';
+
+function collectPaths(routeObjects: RouteObject[]): string[] {
+  return routeObjects.flatMap((route) => [
+    ...(route.path ? [route.path] : []),
+    ...(route.children ? collectPaths(route.children) : []),
+  ]);
+}
+
+describe('SPA route table', () => {
+  it('covers every v1 parity route from the Phase P RFC', () => {
+    expect(collectPaths(routes).sort()).toEqual(
+      [
+        '/',
+        '/admin',
+        '/auth/provider/:provider',
+        '/chat',
+        '/login',
+        '/projects',
+        '/projects/:id',
+        '/register',
+        '/settings',
+        '/tasks',
+      ].sort(),
+    );
+  });
+
+  it('separates guest and authenticated route groups', () => {
+    const guestPaths = collectPaths(routes.at(0)?.children ?? []);
+    const authPaths = collectPaths(routes.at(1)?.children ?? []);
+    expect(guestPaths).toContain('/login');
+    expect(guestPaths).not.toContain('/chat');
+    expect(authPaths).toContain('/chat');
+  });
+});