Fix monthly actuals: allow negative values and fix save/reconcile error

- Remove min={0} from NumberInput to allow negative actuals (refunds/corrections)
- Fix post() in journal-entries service: use id param directly instead of
  RETURNING result which returns [rows, count] in TypeORM QueryRunner
- Handle negative amounts in saveActuals(): negative expense credits the
  expense account, negative income debits the income account

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/modules/journal-entries/journal-entries.service.ts

@@ -140,11 +140,11 @@ export class JournalEntriesService {
       );
     }
 
-    const result = await this.tenant.query(
-      `UPDATE journal_entries SET is_posted = true, posted_by = $1, posted_at = NOW() WHERE id = $2 RETURNING *`,
+    await this.tenant.query(
+      `UPDATE journal_entries SET is_posted = true, posted_by = $1, posted_at = NOW() WHERE id = $2`,
       [userId, id],
     );
-    return this.findOne(result[0].id);
+    return this.findOne(id);
   }
 
   async void(id: string, userId: string, reason: string) {