feat: config module — NVS read/write via Preferences

Add config.h/config.cpp for DeviceConfig NVS persistence using Arduino
Preferences library. Add minimal main.cpp stub. Fix partition table
overlap (nvs 0x6000→0x5000, otadata 0xf000→0xe000) so firmware builds.

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

firmware/src/config.h

@@ -0,0 +1,24 @@
+// firmware/src/config.h
+#pragma once
+#include <Arduino.h>
+
+struct DeviceConfig {
+    String device_id;      // e.g. "dc-0042"
+    String location_id;    // e.g. "retailer-123"
+    String hmac_secret;    // 32-byte hex string
+    String wifi_ssid;
+    String wifi_pass;
+    uint8_t line_offset;   // 0-100, percent of frame height for virtual line
+};
+
+// Load all config from NVS. Returns false if device_id/location_id/hmac_secret missing.
+bool config_load(DeviceConfig& cfg);
+
+// Save WiFi credentials to NVS (called by provisioning after captive portal).
+bool config_save_wifi(const String& ssid, const String& pass);
+
+// Returns true if wifi_ssid is set in NVS.
+bool config_has_wifi();
+
+// Erase WiFi credentials only (factory reset — preserves device_id etc).
+void config_clear_wifi();