[frontend] Build first reference feature in src/features/ as architecture exemplar

[JonnyTran]

Depends on

ARCHITECTURE.md issue and ESLint rules issue.

Problem

The new feature-sliced layout needs a real, non-trivial example before any migration starts. Greenfield, not a port — the goal is to validate ergonomics.

Approach

Pick a small, mostly-self-contained feature to build from scratch under src/features/<name>/ with full api.ts, store.ts (Pinia), types.ts, components, and tests. Suggested candidates (pick one, lowest risk first):

• A read-only schema browser
• Settings / preferences panel
• A new admin metrics view

Acceptance criteria

• Feature ships behind a route in production build.
• Folder follows ARCHITECTURE.md exactly; no useResolve, no class entities.
• Has at least one Pinia store and one component test.
• Reviewed and held up as the canonical example in ARCHITECTURE.md.