Makefile: use esptool directly, add LittleFS and erase targets

PlatformIO's built-in upload breaks on the S2 Mini (1200bps USB
touch reset disconnects the native USB port). All flash targets
now call esptool directly with the correct S2 partition offsets:
bootloader at 0x1000, firmware at 0x10000, LittleFS at 0x290000.

Also adds USB.begin() before Serial.begin() in main.cpp — required
for ESP32-S2 native USB-CDC to initialize properly.

firmware/Makefile

@@ -1,19 +1,43 @@
-.PHONY: build upload uploadfs monitor clean ota
+.PHONY: build buildfs upload uploadfs monitor clean ota flash erase
+
+# Serial port — override with: make upload PORT=/dev/ttyACM0
+PORT ?= /dev/ttyACM2
+BAUD ?= 921600
+BUILD_DIR = .pio/build/lolin_s2_mini
+
+# ESP32-S2 bootloader lives at 0x1000 (not 0x0 like S3/C3)
+BOOTLOADER_OFFSET = 0x1000
+
+# LittleFS partition offset (from partition table)
+FS_OFFSET = 0x290000
+
+ESPTOOL = ~/.platformio/penv/bin/esptool
 
 build:
 	pio run
 
-upload:
-	pio run -t upload
+buildfs:
+	pio run -t buildfs
 
-uploadfs:
-	pio run -t uploadfs
+upload: build
+	$(ESPTOOL) --chip esp32s2 --port $(PORT) --baud $(BAUD) \
+		write_flash \
+		$(BOOTLOADER_OFFSET) $(BUILD_DIR)/bootloader.bin \
+		0x8000 $(BUILD_DIR)/partitions.bin \
+		0x10000 $(BUILD_DIR)/firmware.bin
 
-monitor:
-	pio device monitor
+uploadfs: buildfs
+	$(ESPTOOL) --chip esp32s2 --port $(PORT) --baud $(BAUD) \
+		write_flash $(FS_OFFSET) $(BUILD_DIR)/littlefs.bin
 
 flash: upload uploadfs
 
+monitor:
+	pio device monitor --port $(PORT)
+
+erase:
+	$(ESPTOOL) --chip esp32s2 --port $(PORT) erase_flash
+
 clean:
 	pio run -t clean
 

firmware/src/main.cpp

@@ -185,8 +185,9 @@ bool isOTAEnabled() {
 
 // --- Setup ---
 void setup() {
+    USB.begin();
     Serial.begin(115200);
-    delay(1000);  // Allow USB CDC to initialize
+    delay(2000);  // Allow USB CDC to enumerate on host
 
     Serial.println();
     Serial.println("================================");