refactor(arap): rustfmt the unit's files, and the write-off's SOURCE line (P4b S7)

No behaviour: the files as rustfmt leaves them after the unit's edits
(the commits before this one were made before the formatter ran), and the
Ledger::write_off spec block cites A-5 alone for the direct write-off
method (A-6 is the hand-entry refusal, not this command).

crates/ledger-gui/tests/invoices.rs

@@ -252,8 +252,7 @@ fn a_write_off_shows_on_the_timeline() {
     let mut h = open(&dir, Route::Invoice(inv));
     shot(&mut h, "invoice-written-off");
     assert!(
-        h.query_by_label_contains("Written off, entry #")
-            .is_some(),
+        h.query_by_label_contains("Written off, entry #").is_some(),
         "the timeline shows the write-off"
     );
     // The status chip reads its label ("Written off"), not "late".