fix: flash_device.py — correct nvs_partition_gen module name

esp-idf-nvs-partition-gen installs as esp_idf_nvs_partition_gen,
not nvs_partition_gen.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tools/flash_device.py

@@ -2,7 +2,7 @@
 """
 flash_device.py — Write NVS config to TimerCamera-F over serial.
 
-Requires: pip install esptool nvs-partition-gen
+Requires: pip install esptool esp-idf-nvs-partition-gen
 Usage:
   python flash_device.py \\
     --port /dev/ttyUSB0 \\
@@ -84,7 +84,7 @@ def main():
 
         # Generate NVS binary
         ret = subprocess.run(
-            [sys.executable, "-m", "nvs_partition_gen", "generate",
+            [sys.executable, "-m", "esp_idf_nvs_partition_gen", "generate",
              csv_path, bin_path, NVS_PARTITION_SIZE],
             capture_output=True, text=True
         )