feat(auth): per-user UI language preference (sq default, en)

Add users.language ('sq'|'en', default 'sq'; migration 0003). Returned from
/api/auth/login and /api/auth/me (read from the DB, not the JWT — so changing it
needs no re-login). New PUT /api/auth/language for self-service. Loaded on login
and restored from any booth. Printed tickets stay Albanian (customer-facing).

apps/server/src/routes/auth.ts

@@ -16,6 +16,12 @@ interface LoginBody {
   password: string;
 }
 
+const LANGS = ["sq", "en"] as const;
+type Lang = (typeof LANGS)[number];
+interface LanguageBody {
+  language: Lang;
+}
+
 export async function authRoutes(app: FastifyInstance, db: Db): Promise<void> {
   app.post<{ Body: LoginBody }>("/api/auth/login", async (req, reply) => {
     const { username, password } = req.body ?? {};
@@ -41,7 +47,9 @@ export async function authRoutes(app: FastifyInstance, db: Db): Promise<void> {
       csrf,
     });
     setAuthCookies(reply, token, csrf);
-    return { id: user.id, username: user.username, role: user.role };
+    // `language` is NOT in the JWT (identity/role only) — it's a mutable preference
+    // read from the DB, so changing it needs no token refresh.
+    return { id: user.id, username: user.username, role: user.role, language: user.language };
   });
 
   app.post("/api/auth/logout", async (_req, reply) => {
@@ -49,13 +57,30 @@ export async function authRoutes(app: FastifyInstance, db: Db): Promise<void> {
     return { ok: true };
   });
 
-  // Who am I — used by the SPA to bootstrap session state on load.
+  // Who am I — used by the SPA to bootstrap session state on load. Reads the live
+  // `language` preference from the DB (not the token).
   app.get(
     "/api/auth/me",
     { preHandler: requireRole("admin", "operator", "cashier", "readonly") },
     async (req) => {
       const { sub, username, role } = req.user;
-      return { id: sub, username, role };
+      const row = await db.select().from(users).where(eq(users.id, sub)).get();
+      return { id: sub, username, role, language: row?.language ?? "sq" };
+    },
+  );
+
+  // Change MY own UI language preference (any signed-in user). Persisted to the
+  // users row so it's restored on the next login, from any booth. See i18n.md.
+  app.put<{ Body: LanguageBody }>(
+    "/api/auth/language",
+    { preHandler: requireRole("admin", "operator", "cashier", "readonly") },
+    async (req, reply) => {
+      const language = req.body?.language;
+      if (!language || !LANGS.includes(language)) {
+        return reply.code(400).send({ error: `language must be one of: ${LANGS.join(", ")}` });
+      }
+      await db.update(users).set({ language }).where(eq(users.id, req.user.sub)).run();
+      return { language };
     },
   );
 }