Surface image cleanup and the generic config API

[nfebe]

Two new agent capabilities need UI affordances:

• Image cleanup (enh: Delete stale unused images when new ones are pulled agent#124): a Clean stale images button on each deployment that calls POST /deployments/:name/images/cleanup, a system-wide Prune dangling button in settings or images calling POST /images/cleanup, and a Cleanup toggle on the deploy/rebuild dialog mapping to the cleanup flag on the deploy and pull endpoints.
• Generic config API (Migrate /settings to the generic /config API agent#134): a settings page rendered from GET /config that lists every key with its description, current value, default, and a runtime flag, and a per-row editor that PUTs back. Runtime-applicable keys take effect immediately; the rest should show a Restart required hint. The legacy /settings panels can stay until they are migrated under Migrate /settings to the generic /config API agent#134.