fix: run FTP server in dedicated FreeRTOS task to unblock LVGL

Move ftpSrv.handleFTP() from Arduino loop() into a dedicated FreeRTOS
task pinned to core 0, leaving core 1 free for LVGL display updates.

FTP callbacks now only set volatile flags (ftp_ui_dirty, ftp_sd_dirty)
which the main loop polls to safely update UI and NeoPixel from core 1.
This eliminates the 20-second PASV data connection timeouts caused by
handleFTP() competing with display.update() for CPU time.

Also fix pre-existing BLACK constant build error in display_manager.cpp.

lib/kodedot_bsp/src/display_manager.cpp

@@ -51,7 +51,7 @@ bool DisplayManager::init() {
         if (pct > 100) pct = 100;
         gfx->setBrightness((uint8_t)(((uint16_t)pct * 255 + 50) / 100));
     }
-    gfx->fillScreen(BLACK);
+    gfx->fillScreen((uint16_t)0x0000);
     Serial.println("[DISP] Panel OK");
 
     /* ── LVGL 9 init ───────────────────────────────────────────── */