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/hooks/useFlowPilotSession.ts

@@ -99,9 +99,14 @@ export function useFlowPilotSession(): UseFlowPilotSession {
       setAllSteps([firstStep])
       setCurrentStep(firstStep)
     } catch (e: unknown) {
-      const message = e instanceof Error ? e.message : 'Failed to start session'
+      // Prefer the backend's detail message over the generic axios status string
+      const detail = (e as any)?.response?.data?.detail
+      const message = typeof detail === 'string' ? detail : (e instanceof Error ? e.message : 'Failed to start session')
       setError(message)
-      toast.error(message)
+      // Global axios interceptor already shows a toast for 5xx — skip duplicate
+      if (!(e as any)?.response?.status || (e as any)?.response?.status < 500) {
+        toast.error(message)
+      }
     } finally {
       setIsLoading(false)
     }