fix: FTP crash on download — increase task stack, guard concurrent SD access

- FTP task stack 8KB → 32KB (SimpleFTPServer file I/O needs deep stack)
- Skip calcDirSize when FTP client connected (prevents concurrent SD_MMC
  access from core 0 + core 1 causing null pointer crash)

src/main.cpp

@@ -131,12 +131,11 @@ static void updateSdInfo() {
     if (!sd_ok) return;
     uint64_t total = SD_MMC.totalBytes() / (1024ULL * 1024ULL);
     uint64_t used  = SD_MMC.usedBytes()  / (1024ULL * 1024ULL);
-    Serial.printf("[SD] API: used=%llu MB, total=%llu MB\n", used, total);
-    /* SD_MMC.usedBytes() returns 0 on exFAT / large cards — walk filesystem */
-    if (used == 0 && total > 0) {
+    /* Skip heavy filesystem walk while FTP is active (concurrent SD_MMC
+       access from core 0 + core 1 causes null pointer crashes) */
+    if (used == 0 && total > 0 && !ftp_client_on) {
         uint64_t walked = calcDirSize(SD_MMC, "/");
         used = walked / (1024ULL * 1024ULL);
-        Serial.printf("[SD] Walk: %llu bytes = %llu MB\n", walked, used);
     }
     ui_set_sd(used, total);
 }
@@ -469,7 +468,7 @@ static void initFtp() {
     ftpSrv.begin("kode", "kode");
 
     /* Run FTP in its own task on core 0 so it never blocks LVGL on core 1 */
-    xTaskCreatePinnedToCore(ftpTask, "ftp", 8192, NULL, 2, NULL, 0);
+    xTaskCreatePinnedToCore(ftpTask, "ftp", 32768, NULL, 2, NULL, 0);
 
     Serial.printf("[FTP] FTP server ready on core 0 (PASV IP: %s, data port: 50009)\n",
                   WiFi.localIP().toString().c_str());