add position-size calculator to the recommendation panel

Risk-based sizing on each setup card: shares = floor((account × risk%) /
|entry − stop|), with position value and dollars-at-risk. Account size and
per-trade risk % are editable inline and persisted in localStorage. Flags when
a position would exceed the account (needs margin). Frontend-only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/position.ts

@@ -0,0 +1,37 @@
+export interface PositionSize {
+  shares: number;
+  riskPerShare: number;
+  dollarRisk: number;
+  positionValue: number;
+  /** Position value exceeds the account → needs margin / not affordable in cash. */
+  exceedsAccount: boolean;
+}
+
+/**
+ * Risk-based position sizing. Risk a fixed % of the account per trade; the stop
+ * distance sets how many shares that budget buys:
+ *   shares = floor((account × risk%) / |entry − stop|)
+ * Returns null when inputs are unusable (no account, no risk, zero stop width).
+ */
+export function positionSize(
+  accountSize: number,
+  riskPct: number,
+  entry: number,
+  stop: number,
+): PositionSize | null {
+  const riskPerShare = Math.abs(entry - stop);
+  if (!(accountSize > 0) || !(riskPct > 0) || !(riskPerShare > 0) || !(entry > 0)) {
+    return null;
+  }
+  const budget = accountSize * (riskPct / 100);
+  const shares = Math.floor(budget / riskPerShare);
+  const dollarRisk = shares * riskPerShare;
+  const positionValue = shares * entry;
+  return {
+    shares,
+    riskPerShare,
+    dollarRisk,
+    positionValue,
+    exceedsAccount: positionValue > accountSize,
+  };
+}