feat(evals): add AI Eval Playground for comparing models and prompts

[berkdurmus]

Summary

• Add new AI Eval Playground utility for comparing AI model outputs and prompts
• Implement BYOK (Bring Your Own Key) support for OpenAI, Anthropic, and Google AI
• Build LLM-as-judge scoring system with configurable criteria weights
• Create clean, table-based comparison UI following Linear.app design patterns

Features

Comparison Modes

• Model vs Model: Compare 2-4 models with the same prompt
• Prompt vs Prompt: Compare 2-4 prompt variations with the same model

Supported Providers

Provider	Models
OpenAI	GPT-4o, GPT-4o Mini, GPT-4 Turbo, GPT-3.5 Turbo
Anthropic	Claude 3.5 Sonnet, Claude 3.5 Haiku, Claude 3 Opus
Google AI	Gemini 2.0 Flash, Gemini 1.5 Pro, Gemini 1.5 Flash

LLM-as-Judge Scoring

• 5 evaluation criteria: Accuracy, Relevance, Clarity, Completeness, Conciseness
• Adjustable weight sliders for custom scoring emphasis
• Pairwise comparison with winner detection
• Visual score badges and breakdown bars

Security

• API keys stored in sessionStorage only (cleared on browser close)
• All processing happens client-side
• Keys never leave the browser

Files Added

components/
├── ai-eval/
│   ├── ApiKeyDialog.tsx
│   ├── EvalComparisonGrid.tsx
│   ├── EvalConfigPanel.tsx
│   ├── EvalJudgePanel.tsx
│   ├── EvalModelSelector.tsx
│   ├── EvalResultCell.tsx
│   └── EvalScoreDisplay.tsx
├── hooks/
│   └── useApiKeys.ts
└── utils/
    ├── ai-eval-judge.ts
    ├── ai-eval-providers.ts
    ├── ai-eval-schemas.ts
    └── ai-eval-schemas.test.ts

pages/utilities/
└── ai-eval.tsx