Report each symbol's next-leg stats to the master; drop per-symbol commission

- lib/leg-estimate: an account's next leg sized on every surveyed
  candidate (planLegSize, ranked by rankForTarget) — notional, its
  multiple of the account and the broker's maximum, expected hold, cost
  and win chance. The dashboard's hold estimate and the reporter share it.
- The reporter sends it per DXtrade firm as symbolStats, for one live
  account from the largest prefix group with a leg left to trade.
- The symbol map's per-symbol commission is gone: the firm's applies.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

types.ts

@@ -99,6 +99,8 @@ export interface AccountState {
         hours: number;
         symbol: string;
         notional: number;
+        /** Notional ÷ account size. */
+        multiplier: number;
         volatilityPerDay: number;
         winProbability: number | null;
         /** Expected swap a winning leg pays, added to its take-profit. */