feat: add repayment discounts and interactive net worth trends

Support positive repayment discounts and negative surcharges with exact decimal conversion. Add independent trend toggles, pointer and keyboard date inspection, tests and Chinese documentation.

apps/web/test/quick-entry.test.ts

@@ -1,3 +1,4 @@
+import { repaymentFee } from '../src/repayment';
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
 import {
@@ -76,3 +77,11 @@ test('daily badges expire and filters combine month and day without mutating mar
   );
   assert.equal(p.lastBookedDate, '2026-10-04');
 });
+
+test('repayment discount uses opposite fee sign without decimal precision loss', () => {
+  assert.equal(repaymentFee('5.87654321'), '-5.87654321');
+  assert.equal(repaymentFee('-1.86420975'), '1.86420975');
+  assert.equal(repaymentFee('-0.00000000'), '0');
+  assert.equal(repaymentFee('0'), '0');
+  assert.throws(() => repaymentFee('1e3'));
+});