Phase 0: minimal main.cpp (WiFi + display + LVGL test button) + carry over hardware-independent helpers

- src/main.cpp rewritten for ESP32-S3: Serial + display_init + WiFi
  connect (logic ported from old main.cpp, M5.Lcd prints -> Serial),
  LVGL test button screen to verify touch in Phase 1
- include/text_utils.h, date_utils.h: kept 1:1 (no M5Stack dependency)
- include/secrets.h.example, .gitignore, scripts/{build,deploy}.sh,
  .github/workflows/build.yml: kept 1:1 (hardware-independent)

include/date_utils.h

@@ -0,0 +1,66 @@
+#pragma once
+#include <Arduino.h>
+
+// No NTP/network time used, purely computed from date strings coming from
+// the calendar API itself, as specified.
+namespace DateUtils {
+
+inline const char *weekdayShortDE(int isoWeekday /* 0=Mon..6=Sun */) {
+    static const char *names[7] = {"Mo", "Di", "Mi", "Do", "Fr", "Sa", "So"};
+    if (isoWeekday < 0 || isoWeekday > 6) return "??";
+    return names[isoWeekday];
+}
+
+// Sakamoto's algorithm, returns 0=Monday..6=Sunday
+inline int computeWeekdayMonBased(int year, int month, int day) {
+    static const int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
+    int y = year;
+    if (month < 3) y -= 1;
+    int w = (y + y / 4 - y / 100 + y / 400 + t[month - 1] + day) % 7; // 0=Sun
+    return (w + 6) % 7; // convert to 0=Mon
+}
+
+struct ParsedDateTime {
+    bool valid = false;
+    int year = 0, month = 0, day = 0, hour = 0, minute = 0;
+};
+
+inline ParsedDateTime parseIso(const String &iso) {
+    ParsedDateTime r;
+    if (iso.length() < 10) return r;
+
+    int y = iso.substring(0, 4).toInt();
+    int mo = iso.substring(5, 7).toInt();
+    int d = iso.substring(8, 10).toInt();
+
+    int h = 0, mi = 0;
+    int tIdx = iso.indexOf('T');
+    if (tIdx > 0 && iso.length() >= tIdx + 6) {
+        h = iso.substring(tIdx + 1, tIdx + 3).toInt();
+        mi = iso.substring(tIdx + 4, tIdx + 6).toInt();
+    }
+
+    if (y == 0 || mo == 0 || d == 0) return r;
+
+    r.valid = true;
+    r.year = y; r.month = mo; r.day = d; r.hour = h; r.minute = mi;
+    return r;
+}
+
+// "Mo 3.8. 14:00" for timed events, "Mo 3.8." for all-day events.
+inline String formatShortDE(const String &iso, bool allDay) {
+    ParsedDateTime p = parseIso(iso);
+    if (!p.valid) return iso;
+
+    int wd = computeWeekdayMonBased(p.year, p.month, p.day);
+    String out = String(weekdayShortDE(wd)) + " " + String(p.day) + "." + String(p.month) + ".";
+
+    if (!allDay) {
+        char buf[8];
+        snprintf(buf, sizeof(buf), " %02d:%02d", p.hour, p.minute);
+        out += buf;
+    }
+    return out;
+}
+
+} // namespace DateUtils