firmware: fix C6 filesystem provisioning (LittleFS) + ESP-01S env

The C6 never loaded its /config.json, so it fell back to defaults
(SSID RemoteRig, empty password) and couldn't join Wi-Fi. Two bugs:

- Data file was named esp32-config.json but the firmware reads
  /config.json → renamed to config.json.
- Firmware used SPIFFS while pioarduino's uploadfs builds a LittleFS
  image; the SPIFFS mount then reformatted it empty. Switch the C6 to
  LittleFS (matches the toolchain default and the ESP-01S).

Also:
- log loaded ssid/broker/camera_id on config load (not the password)
- platformio.ini: land the ESP-01S env retarget (board d1_mini ->
  esp01_1m, dout, upload_speed 115200) that was missed in 403e1d9
- committed config.json keeps wifi_password blank; the real value is
  flashed to the device, not stored in git

Verified: C6 loads config and associates (got a DHCP lease). MQTT to
the broker is a separate network issue (hub IP / subnet).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

firmware/src/esp32-mqtt-bridge.cpp

@@ -30,12 +30,12 @@
 #include <WiFiClient.h>
 #include <PubSubClient.h>
 #include <ArduinoJson.h>
-#include <SPIFFS.h>
+#include <LittleFS.h>
 #include <Wire.h>
 #include <U8g2lib.h>
 
 // ────────────────────────────────────────────
-// Configuration (SPIFFS)
+// Configuration (LittleFS)
 // ────────────────────────────────────────────
 
 struct Config {
@@ -54,8 +54,8 @@ struct Config {
 } cfg;
 
 bool loadConfig() {
-  if (!SPIFFS.begin(true)) { Serial.println("[CFG] SPIFFS mount failed"); return false; }
-  File f = SPIFFS.open("/config.json", "r");
+  if (!LittleFS.begin(true)) { Serial.println("[CFG] LittleFS mount failed"); return false; }
+  File f = LittleFS.open("/config.json", "r");
   if (!f) { Serial.println("[CFG] No config — using defaults"); return false; }
 
   JsonDocument doc;
@@ -71,11 +71,14 @@ bool loadConfig() {
   cfg.heartbeat_sec   = doc["heartbeat_interval_sec"] | cfg.heartbeat_sec;
   cfg.bat_raw_min     = doc["bat_raw_min"]   | cfg.bat_raw_min;
   cfg.bat_raw_max     = doc["bat_raw_max"]   | cfg.bat_raw_max;
+  Serial.printf("[CFG] Loaded: ssid=%s broker=%s:%d cam=%s\n",
+                cfg.wifi_ssid.c_str(), cfg.mqtt_broker.c_str(), cfg.mqtt_port,
+                cfg.camera_id.length() ? cfg.camera_id.c_str() : "-");
   return true;
 }
 
 bool saveConfig() {
-  File f = SPIFFS.open("/config.json", "w");
+  File f = LittleFS.open("/config.json", "w");
   if (!f) return false;
   JsonDocument doc;
   doc["wifi_ssid"]     = cfg.wifi_ssid;