feat(ui): showcase-workspace E2 — full preset exposure (all 8 ScenarioPreset cards + expected-skip semantics)

[w7-mgfcode]

Sub-issue of #389 (umbrella: showcase workspace — preserve, restore, and replay showcase runs). Parallel after Foundation (E1 #390).

Purpose

Full preset exposure: surface all 8 ScenarioPreset values (retail_standard, holiday_rush, high_variance, stockout_heavy, new_launches, sparse, demo_minimal, showcase_rich) as guided business-friendly cards in frontend/src/components/demo/ScenarioPicker.tsx. The backend already accepts the full enum (app/features/demo/schemas.py:57) — this is frontend-mostly. Presets that cannot complete every pipeline step (e.g. sparse NaN-WAPE folds, short-window backfill gates) get expected-skip semantics on the card + a runbook entry.

Sub-tasks

To be decomposed via issue-to-subtasks when this epic is picked up.

[w7-mgfcode]

Shipped in PR #395 (merged into dev). All 8 ScenarioPreset values exposed as guided cards with per-preset demo seed profiles (holiday_rush pinned 2024 window) and expected-skip semantics (sparse caveat + runbook #28). All five backend gates + frontend lint/test green; Level-4 real-browser dogfood passed.