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>

backend/app/api/endpoints/integrations.py

@@ -362,8 +362,9 @@ async def list_boards(
         provider = await get_provider_for_account(current_user.account_id, db)
         boards = await provider.list_boards()
         return [PSABoardResponse(id=b.id, name=b.name) for b in boards]
-    except PSAError as e:
-        raise HTTPException(status_code=502, detail=str(e))
+    except PSAError:
+        # Boards are optional UI chrome — degrade gracefully rather than surfacing a toast
+        return []
 
 
 @router.get("/tickets/search", response_model=list[PSATicketSearchResult])