Expose workflow storage mode in the settings UI

[terisuke]

Context

PR #60 added WORKFLOW_STORE_DRIVER=sqlite as an opt-in backend setting, but using only env/Make is not good enough for the web app UX.

Scope

• Show the active workflow storage mode in the settings panel.
• Allow selecting JSON or SQLite from the UI.
• Persist the next-boot setting server-side instead of relying only on localStorage.
• Make it clear that storage mode changes require a server restart.
• Lock the UI when WORKFLOW_STORE_DRIVER is set by the environment.

Acceptance

• Users can choose SQLite from the app settings without editing Makefile or shell env.
• The backend exposes GET/PATCH config endpoints.
• Existing handler coverage remains at or above the gate.

[terisuke]

Closed by PR #62 merged into develop. Storage mode is now visible and configurable from the settings UI; changes persist to data/app_config.json and apply on server restart. Environment-locked WORKFLOW_STORE_DRIVER disables the UI with a clear message.