feat(wio-tracker-l2): internal flash as LittleFS instead of SPIFFS

helpers/esp32/InternalFS.h picks the filesystem: -D ESP32_LITTLEFS mounts
LittleFS on the same "spiffs" partition, otherwise SPIFFS as before. Only the
L2 LVGL envs set it; every other ESP32 board is unchanged. A settings save
drops from ~0.5 s to ~80 ms and the files survive power loss mid-write.
No migration: the first boot formats the partition (fresh identity).

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

solo/wio-tracker-l2/platformio.ini

@@ -70,6 +70,7 @@ build_flags =
   -D NEARBY_MAX=64
   -D HIST_ARCHIVE          ; message history also on the SD card (ui-lvgl/HistoryStore.h)
   -D GPS_SKYVIEW           ; satellites / signal / DOPs for the GPS screen (helpers/sensors/GpsSky.h)
+  -D ESP32_LITTLEFS        ; internal flash as LittleFS, not SPIFFS (helpers/esp32/InternalFS.h)
 build_src_filter = ${Wio_Tracker_L2.build_src_filter}
   +<helpers/esp32/*.cpp>
   +<helpers/ui/LGFXDisplay.cpp>
@@ -89,6 +90,7 @@ lib_deps =
   NetworkClientSecure ; 3.x name of WiFiClientSecure (the header is still there)
   Preferences
   Update              ; Settings > Firmware update
+  LittleFS
 
 ; The same firmware on the old Arduino-ESP32 2.0.17 (ESP-IDF 4.4), kept for
 ; comparison while the 3.x one settles. Not published (the name doesn't end in
@@ -106,4 +108,5 @@ lib_deps =
   HTTPClient
   WiFiClientSecure
   Preferences
+  LittleFS