Phase 2: add Serial-log API verification at boot

Fetch weather/calendar/departures once in setup() after WiFi and dump
parsed fields to Serial, so JSON parsing against the three real
endpoints can be verified on hardware independently of the UI. API
layer itself unchanged (1:1 from m5stack-dashboard).

src/main.cpp

@@ -4,6 +4,9 @@
 #include <secrets.h>
 #include <theme.h>
 #include "display/display_setup.h"
+#include "api/weather_api.h"
+#include "api/calendar_api.h"
+#include "api/departures_api.h"
 
 namespace {
 
@@ -27,6 +30,44 @@ void connectWifi() {
     }
 }
 
+// Phase 2 verification: fetch all three APIs once at boot and dump parsed
+// fields to Serial so JSON parsing can be verified independently of the UI.
+void verifyApisViaSerial() {
+    Serial.println("\n=== Wetter ===");
+    WeatherData w = fetchWeather();
+    Serial.printf("valid=%d temp=%d symbol=%s desc=%s forecast=%u\n",
+                  w.valid, w.current.temperature,
+                  w.current.symbol.c_str(), w.current.description.c_str(),
+                  w.forecast.size());
+    for (size_t i = 0; i < w.forecast.size() && i < 3; ++i) {
+        const auto &f = w.forecast[i];
+        Serial.printf("  [%u] %s %dC %s precip=%.2f/%s\n", i, f.time.c_str(),
+                      f.temperature, f.description.c_str(),
+                      f.precipitationProbability, f.precipitationType.c_str());
+    }
+
+    Serial.println("\n=== Kalender ===");
+    CalendarData c = fetchCalendar();
+    Serial.printf("valid=%d events=%u\n", c.valid, c.events.size());
+    for (size_t i = 0; i < c.events.size() && i < 3; ++i) {
+        const auto &e = c.events[i];
+        Serial.printf("  [%u] %s | %s all_day=%d\n", i, e.summary.c_str(),
+                      e.startAt.c_str(), e.allDay);
+    }
+
+    Serial.println("\n=== MVG ===");
+    DeparturesData d = fetchDepartures();
+    Serial.printf("valid=%d departures=%u\n", d.valid, d.departures.size());
+    for (size_t i = 0; i < d.departures.size() && i < 3; ++i) {
+        const auto &dep = d.departures[i];
+        Serial.printf("  [%u] %s %s -> %s %s delay=%d cancel=%d\n", i,
+                      dep.type.c_str(), dep.line.c_str(),
+                      dep.destination.c_str(), dep.timeStr.c_str(),
+                      dep.delayMin, dep.cancelled);
+    }
+    Serial.println("=== API verification done ===\n");
+}
+
 void btn_event_cb(lv_event_t *e) {
     lv_obj_t *label = (lv_obj_t *)lv_event_get_user_data(e);
     static int n = 0;
@@ -56,6 +97,7 @@ void setup() {
     Serial.println("WT32-SC01 Plus booting...");
     display_init();
     connectWifi();
+    verifyApisViaSerial();
     buildTestScreen();
     Serial.println("Setup done.");
 }