feat(investments): asset classes with target allocation, charts and rebalance

- Asset classes (name, optional target %) managed under Management > Investments,
  chosen per account
- Charts tab: allocation by asset class and currency, value against invested,
  return by investment; off-target classes flagged by the 5/25 rule
- Rebalance tab: what to buy or sell per class to reach the targets
- Investment page: price against average cost with trades, monthly earnings
  and income

frontend/src/js/charts.js

@@ -4,3 +4,23 @@ import zoomPlugin from 'chartjs-plugin-zoom';
 
 Chart.register(SankeyController, Flow, zoomPlugin);
 window.Chart = Chart;
+
+// The theme's text color at a given strength, resolved to a value canvas can draw
+// with. shortcut: read once per chart, so a theme switch shows on the next render.
+window.chartInk = (percent) => {
+  const probe = document.createElement('span');
+  probe.style.color = `color-mix(in oklab, var(--color-base-content) ${percent}%, transparent)`;
+  document.body.appendChild(probe);
+  const color = getComputedStyle(probe).color;
+  probe.remove();
+  return color;
+};
+
+// Formats amounts with the user's separators, as the amounts elsewhere in the app.
+// currency: {prefix, suffix, decimals, decimal, thousand}
+window.chartMoney = (currency) => (v, decimals = currency.decimals) => {
+  const [whole, fraction] = Math.abs(v).toFixed(decimals).split('.');
+  const grouped = whole.replace(/\B(?=(\d{3})+(?!\d))/g, currency.thousand);
+  const number = fraction ? `${grouped}${currency.decimal}${fraction}` : grouped;
+  return `${v < 0 ? '-' : ''}${currency.prefix}${number}${currency.suffix}`;
+};