Add UART OTA upload with A/B partition support.

Firmware buffers 200-byte chunks into 4 KiB blocks for esp_ota_write; goTool
uploads with per-block ACK flow control and larger UART buffers to avoid stalls.

Co-authored-by: Cursor <cursoragent@cursor.com>

main/app_config.h

@@ -12,4 +12,6 @@ typedef struct {
   char running_partition[APP_RUNNING_PARTITION_LABEL_MAX];
 } app_config_t;
 
+const app_config_t *app_config_get(void);
+
 #endif