feat(12.2-04): add the fileupload field with deferred uploads on the form session key

- sessionKey.ts: one 32-byte base64url key per form mount, sent only in headers
- api/files.ts: FileRoutes over the record and child file routes, XHR upload with progress, 401 refresh and retry
- FileuploadField and FileCaptionModal per UI-SPEC section 3: dropzone, image grid, rows, per-item states, client pre-checks, reorder, protected previews
- FormView provides FORM_SESSION, counts pending changes as dirty and sends X-Session-Key on create and update
- fileupload lang keys in en and pl, admin-spa docs note, deferred smoke test, rebuilt dist

admin/src/api/client.ts

@@ -26,6 +26,14 @@ export function onUnauthorized(handler: UnauthorizedHandler | null): void {
   unauthorizedHandler = handler
 }
 
+/**
+ * Reports a session that could not be recovered, for requests that do not
+ * go through the typed client (the upload XMLHttpRequest).
+ */
+export function reportUnauthorized(): void {
+  unauthorizedHandler?.()
+}
+
 /** Registers a listener for successful refreshes (the new access lifetime). */
 export function onRefreshed(handler: RefreshedHandler | null): void {
   refreshedHandler = handler