feat(config): add form-view-enabled feature flag, default off #8013

Task Summary

Add a single feature flag that gates the whole Form View feature, defaulting off, so later tasks can merge without exposing an unfinished feature. Add gui.workflow-workspace.form-view-enabled = false (env-overridable) to gui.conf, read it in GuiConfig (guiWorkflowWorkspaceFormViewEnabled), expose it from /api/config/gui as formViewEnabled (ConfigResource), and add it to the frontend GuiConfigService type/mock. Nothing reads it yet; the final task flips it on.

Task Type

• Refactor / Cleanup
• DevOps / Deployment / CI
• Testing / QA
• Documentation
• Performance
• Other