Phase F: User-defined metric cards + layout export/share (optional)

[delabrcd]

Phase F (optional / later) of the modular UI / widget architecture — see docs/rfc-ui-widget-architecture.md (§3.5, §5 Phase F). Part of the UI-architecture epic.

Problem

Once the registry + server-side layouts exist, power users may want widgets beyond the built-ins and a way to move a dashboard between installs — without introducing a code-execution surface (decision 1: no user-authored code).

Proposal

• A user-defined declarative metric card: pick a dataset field + a formatter + an optional threshold/target — all declarative data, no code.
• Layout export/import (share): serialize/deserialize a DashboardLayout (feasible now layouts are server-side).

Acceptance criteria

• A metric card can be created entirely from the UI (no code), validated against the dataset schema.
• A layout can be exported and imported on another install; round-trips cleanly.
• No code-execution / un-gated surface introduced.

Pointers

• RFC §3.5/§5 Phase F. Builds on Phases A–E.