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/nano/README.md

@@ -19,7 +19,7 @@ The display driver itself is in [lib/BROSE9323](../../lib/BROSE9323/README.md).
 
 ## Tests
 
-`pio test -e native` runs the unit tests in [test/test_nano](../../test/test_nano/test_main.cpp)
+`pio test -e test_nano` runs the unit tests in [test/test_nano](../../test/test_nano/test_main.cpp)
 on the PC. They feed serial data to the firmware and check what it asks the display to do:
 every command, bitmaps, bad input, flip time storage and the full-redraw timing. The
 Arduino core, EEPROM and the display driver are replaced by fakes in