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>

platformio.ini

@@ -4,13 +4,14 @@
 ;   nano_ota   Same Nano firmware, flashed over WiFi through the ESP32
 ;   esp32     ESP32 "brain" (WiFi, NTP clock, Home Assistant) - OTA upload
 ;   esp32_usb  Same ESP32 firmware, flashed over USB (first flash / recovery)
-;   native     Unit tests for the Nano firmware, run on the PC (needs gcc on PATH)
+;   test_nano  Unit tests for the Nano firmware, run on the PC (needs gcc on PATH)
+;   test_esp32 Unit tests for the ESP32 logic, run on the PC
 ;
 ; Build all:        pio run
 ; Build one:        pio run -e nano
 ; Upload one:       pio run -e nano -t upload
 ; Serial monitor:   pio device monitor -e nano
-; Run tests:        pio test -e native
+; Run tests:        pio test -e test_nano -e test_esp32
 ;
 ; https://docs.platformio.org/page/projectconf.html
 
@@ -22,7 +23,7 @@ framework = arduino
 monitor_speed = 115200
 lib_deps =
 	adafruit/Adafruit GFX Library@^1.10.12
-; Tests only run in the native environment, never on the boards
+; Tests only run in the test_* environments on the PC, never on the boards
 test_ignore = *
 
 ; ---------------------------------------------------------------------------
@@ -58,7 +59,7 @@ board = esp32doit-devkit-v1
 build_src_filter = +<esp32/>
 ; ESPSERIAL turns the BROSE9323 driver into a serial client: every draw call is
 ; sent to the Nano as a text command instead of toggling GPIO pins.
-build_flags = -D ESPSERIAL
+build_flags = -D ESPSERIAL -I src/esp32
 lib_deps =
 	${env.lib_deps}
 	https://github.com/robjen/GFX_fonts.git
@@ -74,15 +75,27 @@ upload_protocol = esptool
 upload_port = COM11
 
 ; ---------------------------------------------------------------------------
-; Native - unit tests for the Nano firmware on the PC (test/test_nano)
-; The Arduino core, EEPROM and the display driver are replaced by test/mocks.
+; Unit tests on the PC (pio test -e test_nano -e test_esp32)
 ; ---------------------------------------------------------------------------
-[env:native]
+[native]
 platform = native
 framework =
 lib_deps =
 lib_ignore = BROSE9323
 test_ignore =
 test_build_src = yes
+
+; Nano firmware (test/test_nano). The Arduino core, EEPROM and the display driver are
+; replaced by test/mocks.
+[env:test_nano]
+extends = native
+test_filter = test_nano
 build_src_filter = +<nano/>
 build_flags = -std=gnu++17 -I test/mocks
+
+; ESP32 logic (test/test_esp32): only the modules without Arduino dependencies
+[env:test_esp32]
+extends = native
+test_filter = test_esp32
+build_src_filter = +<esp32/logic/>
+build_flags = -std=gnu++17 -I src/esp32