Add per-slave ESP-NOW OTA progress over UART and fix dashboard updates.

Expose OTA_SLAVE_PROGRESS on the master, track per-slave state during
distribution, run ESP-NOW OTA in a background task so the host can poll
while slaves update, and show master/slave progress in the dashboard
with table layout and faster WebSocket refresh during uploads.

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

main/ota_uart.h

@@ -17,8 +17,14 @@ typedef enum {
   OTA_UART_ST_BLOCK_ACK = 3,
   OTA_UART_ST_SUCCESS = 4,
   OTA_UART_ST_FAILED = 5,
+  /** ESP-NOW slave distribution in progress (see OTA_DIST_* in cmd_ota.c). */
+  OTA_UART_ST_DISTRIBUTING = 6,
 } ota_uart_status_t;
 
+/** OtaStatusPayload.error when status == OTA_UART_ST_DISTRIBUTING. */
+#define OTA_DIST_AGGREGATE 0u
+#define OTA_DIST_PER_SLAVE 1u
+
 typedef enum {
   OTA_FEED_OK = 0,
   OTA_FEED_BLOCK_WRITTEN,