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/src/style.css

@@ -2422,3 +2422,73 @@ textarea,
     top: 225px;
   }
 }
+
+.trend-legend {
+  flex-wrap: wrap;
+  gap: 8px;
+}
+.trend-legend .trend-series {
+  border: 1px solid #dbe4df;
+  background: #fff;
+  padding: 6px 10px;
+  border-radius: 8px;
+  font-size: 13px;
+}
+.trend-legend .trend-series.is-hidden {
+  opacity: 0.55;
+  text-decoration: line-through;
+  background: #f2f5f3;
+}
+.trend-legend .trend-series:focus-visible,
+.trend-plot svg:focus-visible {
+  outline: 2px solid #457bca;
+  outline-offset: 2px;
+}
+.trend-help {
+  font-size: 12px;
+  margin: 8px 0;
+}
+.trend-plot {
+  position: relative;
+}
+.trend-plot .chart {
+  display: block;
+  cursor: crosshair;
+}
+.trend-tooltip {
+  position: absolute;
+  top: 12px;
+  transform: translateX(-50%);
+  pointer-events: none;
+  z-index: 2;
+  width: max-content;
+  max-width: min(280px, 90%);
+  padding: 10px 12px;
+  background: #ffffffee;
+  border: 1px solid #cbd9d2;
+  border-radius: 10px;
+  box-shadow: 0 4px 18px #17374718;
+  font-size: 12px;
+}
+.trend-tooltip dl {
+  display: grid;
+  grid-template-columns: auto auto;
+  gap: 5px 14px;
+  margin: 6px 0 0;
+}
+.trend-tooltip dd {
+  margin: 0;
+  text-align: right;
+  overflow-wrap: anywhere;
+  font-variant-numeric: tabular-nums;
+}
+.trend-tooltip p {
+  margin: 6px 0 0;
+}
+
+.trend-tooltip {
+  left: auto;
+  right: 12px;
+  transform: none;
+  max-width: calc(100% - 24px);
+}