Add ESP32 recovery app with WiFi setup portal

- New flash layout (partitions.csv): factory = recovery app, ota_0/ota_1 =
  clock firmware; esp32_usb uploads to ota_0 so it never overwrites it
- src/recovery: WiFi + OTA + web page only. Opens the password protected
  'FlipDot-Setup' AP with a captive portal when WiFi fails, lets you pick
  another WiFi (stored in NVS, used by both apps), start the clock or stay
- Recovery is started when the clock firmware can't connect to WiFi,
  crashes 3 times, after 3 quick power cycles, from a new Home Assistant
  button, or by the bootloader when no clock firmware works
- Shared NVS store and boot switching in src/esp32/system
- Clock firmware keeps working on the old flash layout (falls back to
  OTA-only safe mode)
- README: recovery app, one-time USB installation; tests: 65 total

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/net/home_assistant.h

@@ -5,7 +5,8 @@
 // Home Assistant integration over MQTT (with discovery):
 //   FlipDot Display Clock     switch: on = clock, off = Nintendo logo
 //   FlipDot Display Power     switch: off = clear the display and switch it off
-//   FlipDot Display Firmware  sensor: version and health, e.g. "2.1.0, ok, 0 crashes"
+//   FlipDot Display Firmware  sensor: version and health, e.g. "2.1.0, ok, 0 crashes, ..."
+//   FlipDot Display Recovery  button: restart into the recovery app (for 10 minutes)
 void homeAssistantBegin(DisplayController &controller);
 void homeAssistantLoop();