feat(slice-2): Landing + Sign-in screens, entry routing, API client (§5.1/5.2/6.5)

Two-step email + one-time-code sign-in behind both doors; pure entry-routing rule unit-
tested with Vitest (§6.8); storefront routing reaches a SLICE-2 signed-in placeholder with
sign-out (PUC-9). check.sh now runs the frontend unit tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/vite.config.ts

@@ -1,5 +1,6 @@
-import { defineConfig } from "vite";
+/// <reference types="vitest/config" />
 import react from "@vitejs/plugin-react";
+import { defineConfig } from "vitest/config";
 
 // The SPA talks to the FastAPI backend on :8000 via a same-origin proxy, so the
 // browser makes no cross-origin calls in dev. Override the target with
@@ -15,4 +16,9 @@ export default defineConfig({
       "/api": apiTarget,
     },
   },
+  // Tests live alongside source; scoping the include to src/ keeps the runner out of
+  // node_modules (and any stray local backup dirs) without naming them.
+  test: {
+    include: ["src/**/*.test.{ts,tsx}"],
+  },
 });