fix: report accurate is_running after trader update to stop redundant start

Live verification of the relaunch (update) branch surfaced this: the update
response omitted is_running, so the launcher fired a redundant start that got
400 'Trader is already running', showing an error toast and skipping the
dashboard navigation even though the relaunch succeeded.

- PUT /traders/:id now reports is_running (a running trader is restarted with
  the new config, so it stays running).
- The already-running rejection carries error_key trader.start.already_running
  and the shared launcher treats it as success (launch is idempotent).

web/src/lib/launch/launchAutopilot.ts

@@ -101,7 +101,16 @@ export async function launchAutopilot(
       : await api.createTrader(traderRequest)
 
     if (!autopilot.is_running) {
-      await api.startTrader(autopilot.trader_id)
+      try {
+        await api.startTrader(autopilot.trader_id)
+      } catch (err) {
+        // Launch is idempotent: the update path restarts a running trader
+        // asynchronously, so a racing "already running" rejection is success.
+        const alreadyRunning =
+          err instanceof ApiError &&
+          err.errorKey === 'trader.start.already_running'
+        if (!alreadyRunning) throw err
+      }
     }
 
     return {