feat(ui): showcase workspace full preset exposure (#391)

[w7-mgfcode]

Summary

Implements E2 — Full Preset Exposure (epic #391, umbrella #389, PRP PRPs/PRP-showcase-workspace-E2-preset-exposure.md).

• Backend — _SCENARIO_SEED_PROFILE extended from 3 to all 8 ScenarioPreset members via a _SeedProfile NamedTuple with an optional calendar-pinned window; step_seed honors the pinned window (holiday_rush → 2024-10-01..2024-12-31, since its HolidayConfig spikes are fixed 2024 dates). All windows ≥ 75 days (historical_backfill gate). sparsity=0.0 preserved so the sparse preset keeps its 50%-missing character (seeder override precedence).
• Frontend — ScenarioPreset union widened to all 8 values (surgical CRLF-preserving edit); ScenarioPicker rewritten from a shadcn <Select> to an aria-pressed card grid (W3C APG button pattern, role="group"), props interface unchanged so showcase.tsx wiring is untouched. Caveat badges: sparse (expected fail/skip — NaN WAPE), holiday_rush (pinned 2024 window), showcase_rich (provider-key skips). Hint line: switching presets only takes effect with Re-seed first.
• No phase-table change — the 5 newly exposed presets run the legacy 11-step flow (backend parametrized test + frontend it.each lockstep test enforce this).
• Docs — API_CONTRACTS.md documents the full 8-value union on POST /demo/run + the WS start frame; RUNBOOKS.md gains showcase entry feat(forecasting): implement baseline model zoo and unified interface #28 (per-preset expected-outcome matrix incl. the sparse NaN-WAPE trap and the re-seed accumulation behavior found during dogfood).

Validation

• ✅ ruff check + ruff format --check clean
• ✅ mypy app/ + pyright app/ (both --strict) — 0 errors
• ✅ Backend unit suite: 1961 passed (incl. new profile-exhaustiveness, pinned-window POST-body, parametrized phase-table tests)
• ✅ Demo slice integration: 10 passed against real Postgres
• ✅ Frontend: pnpm lint (0 errors), pnpm test --run 331 passed (58 files); no NEW tsc -b errors in touched files (24 pre-existing on dev, unchanged)
• ✅ Level-4 real-browser dogfood (Playwright + Chromium): 8 cards render with caveats, default demo_minimal aria-pressed; retail_standard re-seed run green (seed detail 5 stores x 15 products); holiday_rush reset+re-seed run green with /seeder/status exactly 2024-10-01..2024-12-31; demo_minimal restore green, today-anchored window confirmed

Closes #391

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