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Implement the configuration layer that supports the entire cost estimation pipeline: - config/config.go: TOML-based config at ~/.config/cburn/config.toml (XDG-compliant) with sections for general preferences, Admin API credentials, budget tracking, appearance, and per-model pricing overrides. Supports Load/Save with sensible defaults (30-day window, subagents included, Flexoki Dark theme). Admin API key resolution checks ANTHROPIC_ADMIN_KEY env var first, falling back to the config file. - config/pricing.go: Hardcoded pricing table for 8 Claude model variants (Opus 4/4.1/4.5/4.6, Sonnet 4/4.5/4.6, Haiku 3.5/4.5) with per-million-token rates across 5 billing dimensions: input, output, cache_write_5m, cache_write_1h, cache_read, plus long- context overrides (>200K tokens). NormalizeModelName strips date suffixes (e.g., "claude-opus-4-5-20251101" -> "claude-opus-4-5"). CalculateCost and CalculateCacheSavings compute per-call USD costs by multiplying token counts against the pricing table. - config/plan.go: DetectPlan reads ~/.claude/.claude.json to determine the billing plan type. Maps "stripe_subscription" to the Max plan ($200/mo ceiling), everything else to Pro ($100/mo). Used by the budget tab for plan-relative spend visualization. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>