feat: Neural Network Playground — all phases complete

[saagpatel]

Summary

• Phase 1: Core UI — Canvas 2D network graph with weight heatmaps, D3 loss/accuracy charts, training controls, dataset selector, architecture builder, hyperparameter panel
• Phase 2: CNN support, CIFAR-10 dataset, confusion matrix, per-layer activation viewer
• Phase 3: URL state sharing (LZ-string + hash), guided tutorials with auto-triggers, dark/light mode with localStorage persistence, mobile-responsive drawer layout, dynamic D3 + next/dynamic for tab-gated components, README, LICENSE, OG metadata

Test plan

• pnpm tsc --noEmit passes
• pnpm build produces static export in out/
• Share button encodes config in URL hash; pasting URL restores state
• Dark mode toggle persists across page reloads
• Tutorial overlay auto-loads preset and switches tabs on each step
• Mobile: left panel opens as drawer with backdrop at <1024px

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