feat: 添加时区设置功能，默认时区改为 CST-8

- 新增 set_timezone LLM 工具，支持通过对话设置时区
- 新增 set_timezone / timezone_show CLI 命令
- 默认时区从 PST 改为 CST-8（中国标准时间 UTC+8）
- 支持 POSIX 格式和 18 个城市名映射（Asia/Shanghai 等）
- 时区通过 NVS 持久化存储（system_config namespace）
- config_show 中显示当前时区配置
- 更新 changelog.md 和 taolun.md 文档

main/wifi/wifi_manager.c

@@ -2,6 +2,7 @@
 #include "mimi_config.h"
 
 #include <string.h>
+#include <stdbool.h>
 #include <inttypes.h>
 #include "esp_log.h"
 #include "esp_wifi.h"
@@ -12,6 +13,7 @@
 #include "freertos/FreeRTOS.h"
 #include "freertos/task.h"
 #include "freertos/event_groups.h"
+#include "freertos/timers.h"
 
 static const char *TAG = "wifi";
 
@@ -20,6 +22,15 @@ static int s_retry_count = 0;
 static char s_ip_str[16] = "0.0.0.0";
 static bool s_connected = false;
 static bool s_reconnect_enabled = true;
+static TimerHandle_t s_retry_timer = NULL;
+
+static void retry_timer_callback(TimerHandle_t xTimer)
+{
+    (void)xTimer;
+    if (s_reconnect_enabled && !s_connected) {
+        esp_wifi_connect();
+    }
+}
 
 static const char *wifi_reason_to_str(wifi_err_reason_t reason)
 {
@@ -77,9 +88,10 @@ static void event_handler(void *arg, esp_event_base_t event_base,
             }
             ESP_LOGW(TAG, "Disconnected, retry %d/%d in %" PRIu32 "ms",
                      s_retry_count + 1, MIMI_WIFI_MAX_RETRY, delay_ms);
-            vTaskDelay(pdMS_TO_TICKS(delay_ms));
-            esp_wifi_connect();
             s_retry_count++;
+            /* Use timer instead of blocking vTaskDelay in event handler */
+            if (s_retry_timer) xTimerStop(s_retry_timer, 0);
+            xTimerChangePeriod(s_retry_timer, pdMS_TO_TICKS(delay_ms), 0);
         } else {
             ESP_LOGE(TAG, "Failed to connect after %d retries", MIMI_WIFI_MAX_RETRY);
             xEventGroupSetBits(s_wifi_event_group, WIFI_FAIL_BIT);
@@ -98,6 +110,7 @@ static void event_handler(void *arg, esp_event_base_t event_base,
 esp_err_t wifi_manager_init(void)
 {
     s_wifi_event_group = xEventGroupCreate();
+    s_retry_timer = xTimerCreate("wifi_retry", pdMS_TO_TICKS(1000), pdFALSE, NULL, retry_timer_callback);
 
     ESP_ERROR_CHECK(esp_netif_init());
     esp_netif_create_default_wifi_sta();