Add Random symbol option to trade controls

- Symbol dropdown now includes a "Random" option alongside enabled instruments
- runTrade resolves 'Random' to a random enabled instrument once per batch,
  so all accounts in the same tick trade the same symbol
- Import getInstruments in auto-trade.ts to support the resolution

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/page.tsx

@@ -479,6 +479,7 @@ export default function Home() {
                                     className="rounded-lg border border-slate-200 bg-slate-50 px-2.5 py-1.5 text-sm font-mono text-slate-800 focus:outline-none focus:ring-2 focus:ring-blue-500"
                                 >
                                     {enabledSymbols.map((s) => <option key={s} value={s}>{s}</option>)}
+                                    <option value="Random">Random</option>
                                 </select>
                                 <div className="flex rounded-lg border border-slate-200 overflow-hidden text-sm font-semibold">
                                     <button