fix(auth): add Vite dev proxy to fix SameSite=Strict cookie on page reload

Chrome 89+ treats http://localhost and https://localhost as different sites
(schemeful same-site). The refreshToken cookie (SameSite=Strict) was not
sent when the Vite dev server (HTTP) made cross-origin fetch calls to the
.NET backend (HTTPS), causing a 401 on every page reload.

Fix: route /api/* through the Vite dev server proxy so all requests stay
same-origin. The cookie is now always sent and auth sessions survive reloads.

- vite.config.ts: proxy /api → https://localhost:7221 (secure:false for dev cert)
- .env.example: document proxy vs direct mode, set default VITE_API_BASE_URL to empty
- config.ts: accept empty string as valid apiBaseUrl alongside full URLs

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/lib/config.ts

@@ -6,7 +6,8 @@ import { z } from 'zod';
  * once and only warns in development — production trusts the build-time env.
  */
 const configSchema = z.object({
-    apiBaseUrl: z.string().url(),
+    // Empty string = use Vite proxy (same-origin); a full URL = direct mode.
+    apiBaseUrl: z.union([z.literal(''), z.string().url()]),
 });
 
 export type AppConfig = z.infer<typeof configSchema>;