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.cpp

@@ -0,0 +1,48 @@
+// firmware/src/config.cpp
+#include "config.h"
+#include <Preferences.h>
+
+static const char* NS = "doorcounter";
+
+bool config_load(DeviceConfig& cfg) {
+    Preferences prefs;
+    prefs.begin(NS, true); // read-only
+
+    cfg.device_id   = prefs.getString("device_id", "");
+    cfg.location_id = prefs.getString("location_id", "");
+    cfg.hmac_secret = prefs.getString("hmac_secret", "");
+    cfg.wifi_ssid   = prefs.getString("wifi_ssid", "");
+    cfg.wifi_pass   = prefs.getString("wifi_pass", "");
+    cfg.line_offset = (uint8_t)prefs.getUInt("line_offset", 50);
+
+    prefs.end();
+
+    return !cfg.device_id.isEmpty() &&
+           !cfg.location_id.isEmpty() &&
+           !cfg.hmac_secret.isEmpty();
+}
+
+bool config_save_wifi(const String& ssid, const String& pass) {
+    Preferences prefs;
+    prefs.begin(NS, false);
+    bool ok = prefs.putString("wifi_ssid", ssid) &&
+              prefs.putString("wifi_pass", pass);
+    prefs.end();
+    return ok;
+}
+
+bool config_has_wifi() {
+    Preferences prefs;
+    prefs.begin(NS, true);
+    String ssid = prefs.getString("wifi_ssid", "");
+    prefs.end();
+    return !ssid.isEmpty();
+}
+
+void config_clear_wifi() {
+    Preferences prefs;
+    prefs.begin(NS, false);
+    prefs.remove("wifi_ssid");
+    prefs.remove("wifi_pass");
+    prefs.end();
+}