fix: resolve recurring iOS PWA forbidden errors via CSRF response header

iOS Safari in PWA standalone mode unreliably handles cookies, causing
CSRF token desync between client and server after app resume. Previous
fixes (response body token in /auth/me and /auth/login) still left a
window where the token could go stale.

Now the server sends X-CSRF-Token response header on every API response
(via csrfMiddleware), including 403 error responses. The client reads
this header from every response, enabling instant self-healing: a 403
extracts the correct token from the error response itself and retries
without needing an extra /auth/me round-trip.

SW cache bumped to v33 to ensure existing iOS PWA installs pick up the
new client code.

public/sw.js

@@ -12,7 +12,7 @@
  *   API: Immer Netzwerk (kein Caching von Nutzerdaten)
  */
 
-const SHELL_CACHE   = 'oikos-shell-v32';
+const SHELL_CACHE   = 'oikos-shell-v33';
 const PAGES_CACHE   = 'oikos-pages-v28';
 const ASSETS_CACHE  = 'oikos-assets-v27';
 const ALL_CACHES    = [SHELL_CACHE, PAGES_CACHE, ASSETS_CACHE];