Lehrplan: Country-Helper + Landeinstellung live

- App/php/lib/Country.php: Kaskade session → classes → teachers →
  cookie → 'AT', Helper current()/flag()/label()/allCountries()
- App/php/api/country.php: GET + POST /api/country
- country-Spalten in teachers, classes, student_sessions (idempotent)
- Native-<select>-Picker im Header aller öffentlichen Seiten
  (modul-*.php, lehrplan.php, simulationen.php)
- Footer-Links "Andere Länder: 🇨🇭 🇩🇪 🇱🇮"
- lehrplan.php liest Default-Filter jetzt aus Country::current()
- iPad-freundlich: Native Select gibt Wheel-Picker auf iOS,
  Touch-Ziel >=40px

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

App/php/api/country.php

@@ -0,0 +1,52 @@
+<?php
+/**
+ * Country API
+ * -----------
+ * GET  /api/country           → aktuelles Land JSON { country: "AT" }
+ * POST /api/country           → setzt das aktuelle Land
+ *                                Body: { "country": "DE" }
+ *                                Persistiert in Session + Cookie.
+ *
+ * Absichtlich leicht gehalten: Die DB-Felder (teachers/classes/students)
+ * werden *nicht* von dieser API geschrieben — das bleibt dem Profil-Editor
+ * der Lehrperson und dem Admin überlassen. Hier nur der Kurzzeit-Override.
+ */
+
+require_once __DIR__ . '/../config/app.php';
+require_once __DIR__ . '/../lib/Country.php';
+
+header('Content-Type: application/json; charset=utf-8');
+
+$method = $_SERVER['REQUEST_METHOD'] ?? 'GET';
+
+if ($method === 'GET') {
+    echo json_encode([
+        'country' => Country::current(),
+        'flag'    => Country::flag(Country::current()),
+        'label'   => Country::label(Country::current()),
+    ], JSON_UNESCAPED_UNICODE);
+    exit;
+}
+
+if ($method === 'POST') {
+    $body = json_decode(file_get_contents('php://input') ?: '{}', true) ?: [];
+    $code = strtoupper((string)($body['country'] ?? $_POST['country'] ?? ''));
+
+    if (!Country::isValid($code)) {
+        http_response_code(400);
+        echo json_encode(['error' => 'Invalid country code. Expected AT, DE, CH or LI.']);
+        exit;
+    }
+
+    Country::setCurrent($code);
+    echo json_encode([
+        'country' => Country::current(),
+        'flag'    => Country::flag(Country::current()),
+        'label'   => Country::label(Country::current()),
+        'ok'      => true,
+    ], JSON_UNESCAPED_UNICODE);
+    exit;
+}
+
+http_response_code(405);
+echo json_encode(['error' => 'Method not allowed']);