Web assets only use ~489 KB but the default 1.375 MB spiffs
partition wasted the rest. Rebalance to 768 KB spiffs / 1.56 MB
per app slot (was 1.25 MB), giving the tight esp32 build real
headroom (99.6% -> 79.7% flash used) while preserving dual-slot
OTA. Wire the custom layout into both the local PlatformIO build
(new partitions.csv + platformio.ini) and the Arduino-CLI CI/release
pipeline (PartitionScheme=custom, matching littlefs image size and
merge_bin offset).
