feat: add WiFi manager with NVS credentials and auto-reconnect

STA mode, reads SSID/password from NVS, exponential backoff
reconnect (1s-30s), EventGroup for connection state.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main/wifi/wifi_manager.h

@@ -0,0 +1,47 @@
+#pragma once
+
+#include "esp_err.h"
+#include "esp_event.h"
+#include "freertos/FreeRTOS.h"
+#include "freertos/event_groups.h"
+
+#define WIFI_CONNECTED_BIT  BIT0
+#define WIFI_FAIL_BIT       BIT1
+
+/**
+ * Initialize WiFi subsystem (STA mode).
+ * Reads SSID/password from NVS. If not set, waits for serial configuration.
+ */
+esp_err_t wifi_manager_init(void);
+
+/**
+ * Start WiFi connection. Non-blocking, fires events.
+ */
+esp_err_t wifi_manager_start(void);
+
+/**
+ * Block until WiFi is connected or failed.
+ * @param timeout_ms  Max time to wait (portMAX_DELAY for forever)
+ * @return ESP_OK if connected, ESP_ERR_TIMEOUT otherwise
+ */
+esp_err_t wifi_manager_wait_connected(uint32_t timeout_ms);
+
+/**
+ * Check if WiFi is currently connected.
+ */
+bool wifi_manager_is_connected(void);
+
+/**
+ * Save WiFi credentials to NVS.
+ */
+esp_err_t wifi_manager_set_credentials(const char *ssid, const char *password);
+
+/**
+ * Get the current IP address string (or "0.0.0.0" if not connected).
+ */
+const char *wifi_manager_get_ip(void);
+
+/**
+ * Get the event group for WiFi state (WIFI_CONNECTED_BIT / WIFI_FAIL_BIT).
+ */
+EventGroupHandle_t wifi_manager_get_event_group(void);