The transaction export streamed rows straight into an ofstream. A write
that failed partway, on a full disk or removable media, left a truncated
CSV on disk behind a success toast. The CSV formatting also lived inline
in the dialog with no test coverage.
- The CSV is now built in memory and written atomically (temp file plus
rename), so a write failure is reported with an error toast and leaves
no partial file.
- Formatting moves into a reusable helper, and the timestamp conversions
use the thread-safe localtime variants and tolerate a conversion
failure rather than dereferencing a null result.
Tests: field escaping neutralises formula-injection prefixes and quotes
special characters, and the full CSV has the expected header and rows.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
