fix: revert FTP task and calcDirSize (solves timeouts and crashes)

- Remove recursive calcDirSize because it blocks the CPU for several
  seconds during SD card traversal, causing FTP TCP connections to timeout
  before the welcome banner is sent.
- Remove FreeRTOS FTP task entirely. SimpleFTPServer relies on underlying
  Arduino WiFi mechanisms that aren't cleanly isolated into preemptive
  tasks; running it concurrently with LVGL screen updates and SD_MMC
  driver calls caused crashes and starvation.
- Restore tpSrv.handleFTP() to the main loop() as intended by the library.

src/main.cpp

@@ -107,36 +107,10 @@ static void updateLed() {
 // ---------------------------------------------------------------------------
 
 /** Get used / total space on SD card in MiB and push to UI */
-static uint64_t calcDirSize(fs::FS &fs, const char *dirname) {
-    uint64_t total = 0;
-    File root = fs.open(dirname);
-    if (!root || !root.isDirectory()) return 0;
-    File f;
-    while ((f = root.openNextFile())) {
-        if (f.isDirectory()) {
-            /* On ESP32 Arduino 3.x, f.path() returns the full path */
-            String fullPath = String(f.path());
-            f.close();
-            total += calcDirSize(fs, fullPath.c_str());
-        } else {
-            total += f.size();
-            f.close();
-        }
-    }
-    root.close();
-    return total;
-}
-
 static void updateSdInfo() {
     if (!sd_ok) return;
     uint64_t total = SD_MMC.totalBytes() / (1024ULL * 1024ULL);
     uint64_t used  = SD_MMC.usedBytes()  / (1024ULL * 1024ULL);
-    /* 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);
-    }
     ui_set_sd(used, total);
 }
 
@@ -146,7 +120,6 @@ static void updateSdInfo() {
 
 /**
  * Called on FTP connect / disconnect events.
- * NOTE: Runs on the FTP task (core 0) — only set flags here, never call LVGL/UI.
  */
 void ftpCallback(FtpOperation ftpOperation, uint32_t freeSpace, uint32_t totalSpace) {
     switch (ftpOperation) {
@@ -175,7 +148,6 @@ void ftpCallback(FtpOperation ftpOperation, uint32_t freeSpace, uint32_t totalSp
 
 /**
  * Called during file transfer operations.
- * NOTE: Runs on the FTP task (core 0) — only set flags here, never call LVGL/UI.
  */
 void ftpTransferCallback(FtpTransferOperation ftpOperation, const char *name, uint32_t transferredSize) {
     switch (ftpOperation) {
@@ -449,20 +421,7 @@ static void initNtp() {
     Serial.println("[NTP] Warning: time not yet synced (will update in background)");
 }
 
-/** FTP task — runs handleFTP() in a dedicated loop on core 1
- *  NOTE: Must run on core 1 (not core 0) because WiFi runs on core 0
- *  and handleFTP() needs WiFi sockets to work. Running on core 0 at
- *  higher priority starves WiFi, preventing welcome banner from sending.
- */
-static void ftpTask(void *param) {
-    (void)param;
-    for (;;) {
-        ftpSrv.handleFTP();
-        vTaskDelay(pdMS_TO_TICKS(2));  // yield 2ms to let LVGL run
-    }
-}
-
-/** Start FTP server with callbacks and launch dedicated task */
+/** Start FTP server with callbacks */
 static void initFtp() {
     Serial.println("[FTP] Starting FTP server (user: kode, port: 21)...");
     ftpSrv.setCallback(ftpCallback);
@@ -471,11 +430,7 @@ static void initFtp() {
     ftpSrv.setLocalIp(WiFi.localIP());
     ftpSrv.begin("kode", "kode");
 
-    /* Run FTP in its own task on core 1 (same as Arduino) — core 0 is
-       reserved for WiFi/networking stack, running FTP there starves it */
-    xTaskCreatePinnedToCore(ftpTask, "ftp", 32768, NULL, 2, NULL, 1);
-
-    Serial.printf("[FTP] FTP server ready on core 0 (PASV IP: %s, data port: 50009)\n",
+    Serial.printf("[FTP] FTP server ready (PASV IP: %s, data port: 50009)\n",
                   WiFi.localIP().toString().c_str());
     ui_set_ftp("Idle");
 }
@@ -572,7 +527,10 @@ void setup() {
 // ---------------------------------------------------------------------------
 
 void loop() {
-    /* Process FTP status flags (set by callbacks on core 0, consumed here on core 1) */
+    /* Handle FTP client requests */
+    ftpSrv.handleFTP();
+
+    /* Process FTP status flags (set by callbacks) */
     if (ftp_ui_dirty) {
         ftp_ui_dirty = false;
         static const char *ftp_st_str[] = {"Idle", "Connected", "Transferring"};