feat(gui): the journal says a write-off in plain words (P4b S7)

R-B: a new event type gets a describe arm, so the journal's list reads
"Invoice 7 written off, 60.00" and opens the entry, rather than printing
the type at a human. every_journal_event_has_plain_language_text scans
store.rs for the types and fails on any without text.

crates/ledger-gui/src/pages/journal.rs

@@ -812,6 +812,14 @@ pub fn describe(b: &Books, line: &JournalLine) -> Option<Event> {
             format!("Bill paid, {}", amount(p)),
             entry_id.map(Route::Entry),
         ),
+        "invoice.written_off" => {
+            let invoice = p.get("invoice_id").and_then(|v| v.as_u64()).unwrap_or(id);
+            ev(
+                icon::HAND_COINS,
+                format!("Invoice {invoice} written off, {}", amount(p)),
+                entry_id.map(Route::Entry),
+            )
+        }
         "period.created" => ev(
             icon::CALENDAR_PLUS,
             format!("Period {} created", s(p, "name")),