fix: display unlimited quota (∞) when storage_quota_bytes = 0

- Added formatQuotaSize() function to display ∞ for unlimited (0) quota
- Added format_quota_size() Rust function matching JavaScript behavior
- Updated quota defaulting logic from '||' to '== null' check
- This allows 0 (unlimited) to pass through while defaulting to 10 GB
  only when the value is null/undefined
- Call sites now use dedicated formatQuotaSize() or format_quota_size()
  instead of options parameter for cleaner API

src/application/dtos/display_helpers.rs

@@ -367,6 +367,16 @@ pub fn format_file_size(bytes: u64) -> String {
     format!("{} {}", formatted, SIZES[i])
 }
 
+/// Formats a byte count for quota display. When bytes is 0, returns "∞" (unlimited).
+///
+/// Matches the JavaScript `formatQuotaSize()` output.
+pub fn format_quota_size(bytes: u64) -> String {
+    if bytes == 0 {
+        return "∞".to_string();
+    }
+    format_file_size(bytes)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -382,6 +392,15 @@ mod tests {
         assert_eq!(format_file_size(1_073_741_824), "1 GB");
     }
 
+    #[test]
+    fn test_format_quota_size() {
+        // Unlimited quota (0) should show infinity symbol
+        assert_eq!(format_quota_size(0), "∞");
+        // Non-zero values should format normally
+        assert_eq!(format_quota_size(500), "500 Bytes");
+        assert_eq!(format_quota_size(1_073_741_824), "1 GB");
+    }
+
     #[test]
     fn test_icon_class_for_with_extension_fallback() {
         // Specific MIME types