feat(L1): modular widget dashboard — 15 configurable widgets
Replaces monolithic AdminDashboard/ClientDashboard with a per-user
configurable widget grid. 15 widget types: ProductionStats, QueueStatus,
RecentRenders, CostOverview, WorkerStatus, KPISummary, OrderThroughput,
Revenue, ItemStatus, ProcessingTimes, RenderTimeByOutputType,
OutputTypeUsage, TopProducts, OrdersByUser, RenderBackendStats.
DashboardTimeframeContext provides shared timeframe state. Dashboard
config persisted in DB via GET/PUT /api/dashboard/config.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>