fix: FTP storage type (SD_MMC not FFAT), SD used-space walk

- Define FTP_SERVER_NETWORK_TYPE=6 and STORAGE_TYPE=10 directly to bypass
  FtpServerKey.h platform detection that was overriding storage to FFAT
- Fix calcDirSize: properly close files, use name() for recursive path
- Add SD debug logging for used-space calculation
- This fixes file transfers (read/write) failing via FTP

src/main.cpp

@@ -105,15 +105,18 @@ 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 = root.openNextFile();
-    while (f) {
+    File f;
+    while ((f = root.openNextFile())) {
         if (f.isDirectory()) {
-            total += calcDirSize(fs, f.path());
+            String path = f.name();
+            f.close();
+            total += calcDirSize(fs, path.c_str());
         } else {
             total += f.size();
+            f.close();
         }
-        f = root.openNextFile();
     }
+    root.close();
     return total;
 }
 
@@ -121,9 +124,12 @@ 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) {
-        used = calcDirSize(SD_MMC, "/") / (1024ULL * 1024ULL);
+        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);
 }