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/.env.example

@@ -1,4 +1,11 @@
-# Base URL of the SlpModularCms .NET API (Unit 0 backend).
-# The backend must be running with CORS configured to allow this origin
-# and to send the httpOnly refresh-token cookie (credentials: include).
-VITE_API_BASE_URL=http://localhost:5000
+# Base URL of the SlpModularCms .NET API.
+#
+# LOCAL DEV (recommended): leave empty and configure the Vite proxy in
+# vite.config.ts. The proxy forwards /api/* to the backend so all requests
+# stay same-origin — required for the SameSite=Strict refresh-token cookie.
+VITE_API_BASE_URL=
+
+# DIRECT (no proxy): point to the backend URL. CORS must allow this origin
+# with credentials, and SameSite may block the cookie on page reload if the
+# schemes differ (e.g. http frontend → https backend).
+# VITE_API_BASE_URL=https://localhost:7221

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>;

frontend/vite.config.ts

@@ -14,6 +14,17 @@ export default defineConfig({
     },
     server: {
         port: 5173,
+        proxy: {
+            // Route all /api calls through the Vite dev server so the browser
+            // sees a single origin. Without this the refreshToken cookie
+            // (SameSite=Strict) is not sent from http://localhost to
+            // https://localhost (different scheme = cross-site in Chrome 89+).
+            '/api': {
+                target: 'https://localhost:7221',
+                changeOrigin: true,
+                secure: false, // allow self-signed dev cert
+            },
+        },
     },
     test: {
         globals: true,