Dashboard v2 — mock harness for absent endpoints

[thinmintdev]

Parent

#148

What to build

Port the designer's HAL0_DATA mock (from prototype's dash/data.jsx) into a runtime mock layer at ui/src/composables/useMock.js.

Activation modes:

• VITE_MOCK_LEMONADE=1 — full mock for offline dev
• Per-endpoint fallback when real API returns 404 (logs console warning, never crashes)

Mock fixtures are the single source of truth — Playwright apiMock.ts imports the same data so dev + test share one source.

Covers shapes that backend issues #142 (multi-modal slots), #145 (metrics aggregator), #146 (FLM/NPU install) will eventually fulfill. When a real endpoint lands, the store flips back to real data with zero code change in views.

Acceptance criteria

• VITE_MOCK_LEMONADE=1 npm run dev boots dashboard with NO backend running
• Mock fixtures importable from Playwright specs (one source of truth)
• Per-endpoint 404 fallback logs console warning + does not crash
• Documented in docs/dev/web-ui-mocks.md
• Existing Playwright specs adapted to new shared-fixture import path

Blocked by

• Dashboard v2 — Pinia store skeleton (Lemonade + Banners + Toasts) #165