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/display/frames.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include "BROSE9323.h"
+#include "logic/display_controller.h"
+
+// Draws the screens on the flipdot display. Each method sends one complete frame.
+class FlipdotFrames : public Frames
+{
+public:
+	explicit FlipdotFrames(BROSE9323 &display);
+
+	// Pikachu with a thought bubble showing HH:MM (yawning in the evening)
+	void clock(int hour, int minute) override;
+	// Nintendo logo
+	void logo() override;
+	void blank() override;
+	// A line of text, e.g. while connecting
+	void message(const char *text);
+
+private:
+	BROSE9323 &_display;
+};