feat(web): i18n with react-i18next — Albanian default, English second

Add react-i18next with two key-parity-checked catalogs (sq default/fallback, en).
Active language driven by the logged-in user's stored preference (applied after
/me resolves); SQ/EN toggle in the header persists via PUT /api/auth/language.
Translate the booth (screen, pay/exit modal, active sessions, snapshots, status),
Login, ShiftControl, SiteSettings, PermitManager, TariffComposer.

SetupWizard deferred (its content is server-provided; needs backend catalog i18n).

apps/web/src/api.ts

@@ -45,10 +45,13 @@ export class ApiError extends Error {
 // --- Auth -----------------------------------------------------------------
 
 export type Role = "admin" | "operator" | "cashier" | "readonly";
+export type Lang = "sq" | "en";
 export interface SessionUser {
   id: string;
   username: string;
   role: Role;
+  /** Preferred UI language (loaded from the server on login). */
+  language: Lang;
 }
 
 export function login(username: string, password: string): Promise<SessionUser> {
@@ -62,6 +65,11 @@ export function logout(): Promise<{ ok: boolean }> {
   return apiFetch("/api/auth/logout", { method: "POST" });
 }
 
+/** Persist the current user's UI language preference (restored on next login). */
+export function setLanguagePref(language: Lang): Promise<{ language: Lang }> {
+  return apiFetch("/api/auth/language", { method: "PUT", body: JSON.stringify({ language }) });
+}
+
 /** Returns the current user, or null if not authenticated. */
 export async function fetchMe(): Promise<SessionUser | null> {
   try {