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>

src/esp32/config.h

@@ -0,0 +1,69 @@
+#pragma once
+
+// All ESP32 settings. Credentials are in secrets.h (see secrets.example.h).
+// Kept free of Arduino types so the logic modules can be tested on the PC.
+
+#include <stdint.h>
+
+// ---------------------------------------------------------------------------
+// Pins
+// ---------------------------------------------------------------------------
+
+const int LED_PIN = 2;    // built-in LED of the ESP32 DevKit v1
+const int RELAY_PIN = 13; // powers the panels and the Nano (LOW = on)
+
+// Link to the Nano on UART2. Serial (UART0) stays free for USB / debug output.
+const int NANO_RX_PIN = 16; // wired to the Nano TX (through a 5V -> 3.3V divider)
+const int NANO_TX_PIN = 17; // wired to the Nano RX
+
+// Optional: GPIO that resets the Nano through an NPN transistor (HIGH = reset), -1 = not wired.
+// Needed to flash a Nano with the Optiboot bootloader, which doesn't start after a power cycle.
+const int NANO_RESET_PIN = -1;
+
+// ---------------------------------------------------------------------------
+// Display and Nano
+// ---------------------------------------------------------------------------
+
+const uint8_t DISPLAY_WIDTH = 84;
+const uint8_t DISPLAY_HEIGHT = 19;
+const uint8_t PANEL_WIDTH = 28;
+
+const uint16_t FLIP_TIME = 225; // flip pulse length in us, sent to the Nano at boot
+
+const uint32_t NANO_BAUD = 115200;
+// Max wait for the Nano's READY after power on (also the wait when Nano TX is not wired to our RX)
+const uint32_t NANO_BOOT_TIMEOUT = 3000;
+
+// After a frame is cleared, wait this long before cutting the power, so the dots can flip
+const uint32_t CLEAR_BEFORE_OFF_TIME = 1000;
+// Power off this long after the display is no longer needed (night, standby, logo)
+const uint32_t POWER_OFF_DELAY = 30000;
+
+// ---------------------------------------------------------------------------
+// Flashing the Nano over WiFi (see README.md, pio run -e nano_ota -t upload)
+// ---------------------------------------------------------------------------
+
+const uint16_t NANO_FLASH_PORT = 2323;
+// The bootloader baud rate is chosen by the uploader (57600 old bootloader, 115200 Optiboot)
+const uint32_t NANO_POWER_OFF_TIME = 1000; // relay off time to make sure the Nano resets
+const uint32_t NANO_RESET_DELAY = 200;     // after power on, before the reset pulse
+const uint32_t NANO_RESET_PULSE = 50;
+
+// ---------------------------------------------------------------------------
+// Network, time and Home Assistant
+// ---------------------------------------------------------------------------
+
+#define HOSTNAME "FlipDotDisplay"
+
+// Europe/Amsterdam
+#define TIMEZONE "CET-1CEST,M3.5.0,M10.5.0/3"
+#define NTP_SERVER "pool.ntp.org"
+
+// After a crash or unexpected reset, only run OTA for this long before starting the
+// main code, so a firmware that crashes at boot can still be replaced
+const uint32_t OTA_PANIC_WINDOW = 15000;
+
+// Home Assistant device id (do not change: Home Assistant's entities are linked to it)
+const uint8_t HA_DEVICE_ID[] = {0x34, 0x86, 0x5d, 0x3a, 0x08, 0x28};
+#define HA_DEVICE_NAME "FlipDotDisplay"
+#define HA_SOFTWARE_VERSION "2.0.0"