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/proxy/http_proxy.h

@@ -14,6 +14,16 @@ esp_err_t http_proxy_init(void);
  */
 bool http_proxy_is_enabled(void);
 
+/**
+ * Save proxy host and port to NVS.
+ */
+esp_err_t http_proxy_set(const char *host, uint16_t port);
+
+/**
+ * Remove proxy config from NVS.
+ */
+esp_err_t http_proxy_clear(void);
+
 /* ── Proxied HTTPS connection ─────────────────────────────────── */
 
 typedef struct proxy_conn proxy_conn_t;