fix(rls): add account_id to AISessionStep creations, fix boards toast

- flowpilot_engine: pass account_id at all 5 AISessionStep instantiation
  sites (_create_step_from_parsed x3, briefing step, status update step).
  Phase 4 RLS blocked every INSERT with NULL account_id — this broke all
  new FlowPilot sessions since the Phase 4 migration was applied.
- integrations: list_boards returns [] on PSAError instead of 502, stopping
  the spurious 'Server error' toast on dashboard load (boards are optional).
- client.ts: 5xx global toast now shows backend detail when available.
- useFlowPilotSession: startSession extracts backend detail for error state;
  suppresses duplicate toast for 5xx (global interceptor already handles it).

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

frontend/src/api/client.ts

@@ -49,9 +49,9 @@ function handleGlobalError(error: AxiosError) {
     return
   }
 
-  // Server errors (5xx)
+  // Server errors (5xx) — show backend detail when available, else generic message
   if (status >= 500) {
-    toast.error('Server error - please try again later')
+    toast.error(detail || 'Server error - please try again later')
     return
   }
 }