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/logic/display_controller.h

@@ -0,0 +1,68 @@
+#pragma once
+
+#include <stdint.h>
+
+#include "logic/power_timer.h"
+
+// Power for the panels and the Nano (implemented by NanoLink)
+class DisplayPower
+{
+public:
+	virtual bool isOn() = 0;
+	// Switch on and wait until the Nano is ready
+	virtual void on() = 0;
+	virtual void off() = 0;
+};
+
+// The screens the display can show (implemented by FlipdotFrames)
+class Frames
+{
+public:
+	virtual void clock(int hour, int minute) = 0;
+	virtual void logo() = 0;
+	virtual void blank() = 0;
+};
+
+// Decides what the display shows and when it is powered, based on the time and the two
+// Home Assistant switches. A frame is only sent when it changes. Pure logic, tested in
+// test/test_esp32.
+class DisplayController
+{
+public:
+	DisplayController(DisplayPower &power, Frames &frames);
+
+	// Home Assistant "FlipDot Display Clock" switch: off shows the logo instead of the clock
+	void setClockEnabled(bool enabled);
+	// Home Assistant "FlipDot Display Power" switch: off clears the display and powers it off
+	void setPowerEnabled(bool enabled);
+
+	bool clockEnabled() const;
+	bool powerEnabled() const;
+
+	// Call often (every loop). time_valid is false until the time is synced over NTP.
+	void update(uint32_t now, bool time_valid, int hour, int minute);
+
+private:
+	enum class Screen
+	{
+		Unknown, // nothing sent since the last power on
+		Clock,
+		Logo,
+		Blank,
+	};
+
+	DisplayPower &_power;
+	Frames &_frames;
+	PowerTimer _off_timer;
+
+	bool _clock_enabled = true;
+	bool _power_enabled = true;
+
+	Screen _screen = Screen::Unknown;
+	int _shown_hour = -1;
+	int _shown_minute = -1;
+
+	void _showClock(int hour, int minute);
+	void _showLogo();
+	void _showBlank();
+};