Banana's "Text file with column headers" import (Actions → Import into
accounting) requires a TAB-separated .txt with unquoted values — picpeak was
emitting a comma-separated, quoted .csv, which won't even show in Banana's
*.txt file picker, let alone parse into columns.
- ledgerService.exportPostings: the `banana` format now serialises TAB-separated
with no quoting, .txt extension, text/plain content-type. generic + bexio stay
comma-CSV (RFC 4180). Tab/newline chars in a cell are collapsed to spaces.
- Frontend ledger.service: download filename uses .txt for banana.
- Tests updated for the new banana shape (tab header, .txt, text/plain).
The column names already matched Banana's NameXml; only the serialisation was
wrong. bexio left as comma-CSV (verify against bexio's import spec separately).