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>

partitions.csv

@@ -0,0 +1,12 @@
+# ESP32 flash layout (4 MB). See README.md "Recovery app".
+#
+#   factory  recovery app (src/recovery): WiFi setup portal + OTA, never changed by OTA
+#   ota_0/1  clock firmware (src/esp32), updated over OTA in turns, with rollback
+#
+# Name,   Type, SubType,  Offset,   Size
+nvs,      data, nvs,      0x9000,   0x5000
+otadata,  data, ota,      0xe000,   0x2000
+factory,  app,  factory,  0x10000,  0x100000
+ota_0,    app,  ota_0,    0x110000, 0x170000
+ota_1,    app,  ota_1,    0x280000, 0x170000
+coredump, data, coredump, 0x3F0000, 0x10000