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)

.github/workflows/build.yml

@@ -0,0 +1,24 @@
+name: PlatformIO CI
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+      - uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cache/pip
+            ~/.platformio/.cache
+          key: ${{ runner.os }}-pio
+      - name: Install PlatformIO Core
+        run: pip install --upgrade platformio
+      - name: Generate placeholder secrets.h for CI build
+        run: cp include/secrets.h.example include/secrets.h
+      - name: Build PlatformIO Project
+        run: pio run