Restructure ESP32 firmware into modules, fix bugs, add tests

Structure (src/esp32/):
- logic/: DisplayController, schedule, power timer, LED pattern; pure C++
- display/: FlipdotFrames and bitmaps
- nano_link/: NanoLink (relay, serial, READY, bootloader) and flash bridge
- net/: WiFi/OTA/panic window, Home Assistant
- config.h with all settings; libs.h (globals in a header) removed

Bug fixes:
- Home Assistant switches now report their state back and start as ON
- loop() no longer blocks: WiFi.waitForConnectResult() (up to 60 s
  without WiFi) and getLocalTime() (5 s before NTP sync) were called
  every iteration
- Power switch off now powers off after 1 s even when a 30 s countdown
  is already running
- millis() overflow safe power-off timer; no frame catch-up bursts
- No garbage time string before NTP sync; 'Connecting...' stays instead
- Frames are only sent when the screen changes instead of every second
- Reset reasons are logged once instead of every 100 ms
- Removed dead code (unused fonts, WiFiMulti, rotated bitmap, ...)

Tests: new test_esp32 environment (26 tests); native renamed to test_nano

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/nano_net_upload.py

@@ -232,7 +232,7 @@ def upload(hex_path, host, port, bauds):
                 raise NoBootloader(
                     f"no answer from the Nano bootloader at {', '.join(map(str, bauds))} baud. "
                     "Check the Nano TX -> ESP32 GPIO 16 wire. A Nano with Optiboot also needs "
-                    "the reset wire (nano_reset in src/esp32/libs.h).")
+                    "the reset wire (NANO_RESET_PIN in src/esp32/config.h).")
             flash(sock, image, baud)
             print("Done. The ESP32 restarts the Nano with the new firmware.")
             return