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)

scripts/build.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+set -euo pipefail
+cd "$(dirname "$0")/.."
+VENV_DIR=".venv-platformio"
+if [ ! -d "$VENV_DIR" ]; then
+    echo "Creating local virtualenv in $VENV_DIR ..."
+    python3 -m venv "$VENV_DIR"
+    "$VENV_DIR/bin/pip" install --upgrade pip
+    "$VENV_DIR/bin/pip" install platformio
+fi
+echo "Running build (no upload) ..."
+"$VENV_DIR/bin/pio" run
+echo "Build finished successfully."