Studio settings: runtime inventory, storage health, settings workspace #1750

Parent: #1736. Stack layers: plan/studio-settings, studio/settings.

Scope

• BFF: GET /api/v1/settings/runtime as a read-only allowlisted projection (build id, server implementation, provider display endpoint, model and provider inventory with synthesised provider rows, fixed deployment-managed management flags), calling server.info only under the server_info feature and models.list only under modelSelection; GET /api/v1/storage/health gated on storageHealth. Both behind the session gate; no mutations; never a credential or raw endpoint.
• Web: the eight-section settings workspace (profile with sign-out, agent, appearance, memory, learning, models, storage, about); chat's model pickers re-wired to the live inventory; "Settings" joins the nav.

Contract

docs/acceptance/studio-settings.md (Bounded).

Verification

89 server and 227 web Vitest proofs; smoke against mecated --mock (runtime settings served with exactly the contract's keys; degraded paths for models and storage).