fix(auth): configurable SameSite cookie for cross-origin dev setup

Chrome 89+ schemeful same-site treats http://localhost and https://localhost
as different sites, blocking SameSite=Strict cookies on cross-origin fetch
(e.g. Vite on port 5173, API on port 7221).

Fix: make CookieSameSite configurable per environment in JwtSettings.
- Default: Strict (production)
- appsettings.Development.json: None (allows cross-origin cookie in dev)
- When SameSite=None, Secure is always forced (browser requirement)

Revert the earlier Vite proxy approach in favour of this backend config.
VITE_API_BASE_URL remains a freely configurable URL in .env.local.

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

frontend/src/lib/config.ts

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