feat: add configuration system with pricing tables and plan detection

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>

internal/config/pricing.go

@@ -0,0 +1,135 @@
+package config
+
+import "strings"
+
+// ModelPricing holds per-million-token prices for a model.
+type ModelPricing struct {
+	InputPerMTok        float64
+	OutputPerMTok       float64
+	CacheWrite5mPerMTok float64
+	CacheWrite1hPerMTok float64
+	CacheReadPerMTok    float64
+	// Long context overrides (>200K input tokens)
+	LongInputPerMTok  float64
+	LongOutputPerMTok float64
+}
+
+// DefaultPricing maps model base names to their pricing.
+var DefaultPricing = map[string]ModelPricing{
+	"claude-opus-4-6": {
+		InputPerMTok: 5.00, OutputPerMTok: 25.00,
+		CacheWrite5mPerMTok: 6.25, CacheWrite1hPerMTok: 10.00, CacheReadPerMTok: 0.50,
+		LongInputPerMTok: 10.00, LongOutputPerMTok: 37.50,
+	},
+	"claude-opus-4-5": {
+		InputPerMTok: 5.00, OutputPerMTok: 25.00,
+		CacheWrite5mPerMTok: 6.25, CacheWrite1hPerMTok: 10.00, CacheReadPerMTok: 0.50,
+		LongInputPerMTok: 10.00, LongOutputPerMTok: 37.50,
+	},
+	"claude-opus-4-1": {
+		InputPerMTok: 15.00, OutputPerMTok: 75.00,
+		CacheWrite5mPerMTok: 18.75, CacheWrite1hPerMTok: 30.00, CacheReadPerMTok: 1.50,
+		LongInputPerMTok: 30.00, LongOutputPerMTok: 112.50,
+	},
+	"claude-opus-4": {
+		InputPerMTok: 15.00, OutputPerMTok: 75.00,
+		CacheWrite5mPerMTok: 18.75, CacheWrite1hPerMTok: 30.00, CacheReadPerMTok: 1.50,
+		LongInputPerMTok: 30.00, LongOutputPerMTok: 112.50,
+	},
+	"claude-sonnet-4-6": {
+		InputPerMTok: 3.00, OutputPerMTok: 15.00,
+		CacheWrite5mPerMTok: 3.75, CacheWrite1hPerMTok: 6.00, CacheReadPerMTok: 0.30,
+		LongInputPerMTok: 6.00, LongOutputPerMTok: 22.50,
+	},
+	"claude-sonnet-4-5": {
+		InputPerMTok: 3.00, OutputPerMTok: 15.00,
+		CacheWrite5mPerMTok: 3.75, CacheWrite1hPerMTok: 6.00, CacheReadPerMTok: 0.30,
+		LongInputPerMTok: 6.00, LongOutputPerMTok: 22.50,
+	},
+	"claude-sonnet-4": {
+		InputPerMTok: 3.00, OutputPerMTok: 15.00,
+		CacheWrite5mPerMTok: 3.75, CacheWrite1hPerMTok: 6.00, CacheReadPerMTok: 0.30,
+		LongInputPerMTok: 6.00, LongOutputPerMTok: 22.50,
+	},
+	"claude-haiku-4-5": {
+		InputPerMTok: 1.00, OutputPerMTok: 5.00,
+		CacheWrite5mPerMTok: 1.25, CacheWrite1hPerMTok: 2.00, CacheReadPerMTok: 0.10,
+		LongInputPerMTok: 2.00, LongOutputPerMTok: 7.50,
+	},
+	"claude-haiku-3-5": {
+		InputPerMTok: 0.80, OutputPerMTok: 4.00,
+		CacheWrite5mPerMTok: 1.00, CacheWrite1hPerMTok: 1.60, CacheReadPerMTok: 0.08,
+		LongInputPerMTok: 1.60, LongOutputPerMTok: 6.00,
+	},
+}
+
+// NormalizeModelName strips date suffixes from model identifiers.
+// e.g., "claude-opus-4-5-20251101" -> "claude-opus-4-5"
+func NormalizeModelName(raw string) string {
+	// Models can have date suffixes like -20251101 (8 digits)
+	// Strategy: try progressively shorter prefixes against the pricing table
+	if _, ok := DefaultPricing[raw]; ok {
+		return raw
+	}
+
+	// Strip last segment if it looks like a date (all digits)
+	parts := strings.Split(raw, "-")
+	if len(parts) >= 2 {
+		last := parts[len(parts)-1]
+		if isAllDigits(last) && len(last) >= 8 {
+			candidate := strings.Join(parts[:len(parts)-1], "-")
+			if _, ok := DefaultPricing[candidate]; ok {
+				return candidate
+			}
+		}
+	}
+
+	return raw
+}
+
+func isAllDigits(s string) bool {
+	for _, c := range s {
+		if c < '0' || c > '9' {
+			return false
+		}
+	}
+	return len(s) > 0
+}
+
+// LookupPricing returns the pricing for a model, normalizing the name first.
+// Returns zero pricing and false if the model is unknown.
+func LookupPricing(model string) (ModelPricing, bool) {
+	normalized := NormalizeModelName(model)
+	p, ok := DefaultPricing[normalized]
+	return p, ok
+}
+
+// CalculateCost computes the estimated cost in USD for a single API call.
+func CalculateCost(model string, inputTokens, outputTokens, cache5m, cache1h, cacheRead int64) float64 {
+	pricing, ok := LookupPricing(model)
+	if !ok {
+		return 0
+	}
+
+	// Use standard context pricing (long context detection would need total
+	// input context size which we don't have per-call; standard is the default)
+	cost := float64(inputTokens) * pricing.InputPerMTok / 1_000_000
+	cost += float64(outputTokens) * pricing.OutputPerMTok / 1_000_000
+	cost += float64(cache5m) * pricing.CacheWrite5mPerMTok / 1_000_000
+	cost += float64(cache1h) * pricing.CacheWrite1hPerMTok / 1_000_000
+	cost += float64(cacheRead) * pricing.CacheReadPerMTok / 1_000_000
+
+	return cost
+}
+
+// CalculateCacheSavings computes how much the cache reads saved vs full input pricing.
+func CalculateCacheSavings(model string, cacheReadTokens int64) float64 {
+	pricing, ok := LookupPricing(model)
+	if !ok {
+		return 0
+	}
+	// Cache reads at cache rate vs what they would have cost at full input rate
+	fullCost := float64(cacheReadTokens) * pricing.InputPerMTok / 1_000_000
+	actualCost := float64(cacheReadTokens) * pricing.CacheReadPerMTok / 1_000_000
+	return fullCost - actualCost
+}