feat: restore NVS runtime config with CLI commands (NVS > build-time priority)

Bring back two-layer configuration: build-time defaults from mimi_secrets.h
with runtime NVS overrides via serial CLI. NVS values take highest priority
so a pre-flashed board can be reconfigured anywhere with just a USB cable.

Restored CLI commands: wifi_set, set_tg_token, set_api_key, set_model,
set_proxy, clear_proxy, set_search_key. Added new commands: config_show
(displays all config with sensitive fields masked), config_reset (clears
all NVS overrides), and help (lists all commands).

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

main/wifi/wifi_manager.h

@@ -35,6 +35,11 @@ bool wifi_manager_is_connected(void);
  */
 const char *wifi_manager_get_ip(void);
 
+/**
+ * Save WiFi credentials to NVS.
+ */
+esp_err_t wifi_manager_set_credentials(const char *ssid, const char *password);
+
 /**
  * Get the event group for WiFi state (WIFI_CONNECTED_BIT / WIFI_FAIL_BIT).
  */