feat: add claude.ai API client and session key configuration

Introduce a client for fetching subscription data from the claude.ai web
API, enabling rate-limit monitoring and overage tracking in the dashboard.

New package internal/claudeai:
- Client authenticates via session cookie (sk-ant-sid... prefix validated)
- FetchAll() retrieves orgs, usage windows, and overage in one call,
  returning partial data when individual requests fail
- FetchOrganizations/FetchUsage/FetchOverageLimit for granular access
- Defensive utilization parsing handles polymorphic API responses: int
  (75), float (0.75 or 75.0), and string ("75%" or "0.75"), normalizing
  all to 0.0-1.0 range
- 10s request timeout, 1MB body limit, proper status code handling
  (401/403 -> ErrUnauthorized, 429 -> ErrRateLimited)

Types (claudeai/types.go):
- Organization, UsageResponse, UsageWindow (raw), OverageLimit
- SubscriptionData (TUI-ready aggregate), ParsedUsage, ParsedWindow

Config changes (config/config.go):
- Add ClaudeAIConfig struct with session_key and org_id fields
- Add GetSessionKey() with CLAUDE_SESSION_KEY env var fallback
- Fix directory permissions 0o755 -> 0o750 (gosec G301)
- Fix Save() to propagate encoder errors before closing file

internal/claudeai/client.go

@@ -0,0 +1,234 @@
+// Package claudeai provides a client for fetching subscription and usage data from claude.ai.
+package claudeai
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"strconv"
+	"strings"
+	"time"
+)
+
+const (
+	baseURL        = "https://claude.ai/api"
+	requestTimeout = 10 * time.Second
+	maxBodySize    = 1 << 20 // 1 MB
+	keyPrefix      = "sk-ant-sid"
+)
+
+var (
+	// ErrUnauthorized indicates the session key is expired or invalid.
+	ErrUnauthorized = errors.New("claudeai: unauthorized (session key expired or invalid)")
+	// ErrRateLimited indicates the API rate limit was hit.
+	ErrRateLimited = errors.New("claudeai: rate limited")
+)
+
+// Client fetches subscription data from the claude.ai web API.
+type Client struct {
+	sessionKey string
+	http       *http.Client
+}
+
+// NewClient creates a client for the given session key.
+// Returns nil if the key is empty or has the wrong prefix.
+func NewClient(sessionKey string) *Client {
+	sessionKey = strings.TrimSpace(sessionKey)
+	if sessionKey == "" {
+		return nil
+	}
+	if !strings.HasPrefix(sessionKey, keyPrefix) {
+		return nil
+	}
+	return &Client{
+		sessionKey: sessionKey,
+		http:       &http.Client{},
+	}
+}
+
+// FetchAll fetches orgs, usage, and overage for the first organization.
+// Partial data is returned even if some requests fail.
+func (c *Client) FetchAll(ctx context.Context) *SubscriptionData {
+	result := &SubscriptionData{FetchedAt: time.Now()}
+
+	orgs, err := c.FetchOrganizations(ctx)
+	if err != nil {
+		result.Error = err
+		return result
+	}
+	if len(orgs) == 0 {
+		result.Error = errors.New("claudeai: no organizations found")
+		return result
+	}
+
+	result.Org = orgs[0]
+	orgID := orgs[0].UUID
+
+	// Fetch usage and overage independently — partial results are fine
+	usage, usageErr := c.FetchUsage(ctx, orgID)
+	if usageErr == nil {
+		result.Usage = usage
+	}
+
+	overage, overageErr := c.FetchOverageLimit(ctx, orgID)
+	if overageErr == nil {
+		result.Overage = overage
+	}
+
+	// Surface first non-nil error for status display
+	if usageErr != nil {
+		result.Error = usageErr
+	} else if overageErr != nil {
+		result.Error = overageErr
+	}
+
+	return result
+}
+
+// FetchOrganizations returns the list of organizations for this session.
+func (c *Client) FetchOrganizations(ctx context.Context) ([]Organization, error) {
+	body, err := c.get(ctx, "/organizations")
+	if err != nil {
+		return nil, err
+	}
+
+	var orgs []Organization
+	if err := json.Unmarshal(body, &orgs); err != nil {
+		return nil, fmt.Errorf("claudeai: parsing organizations: %w", err)
+	}
+	return orgs, nil
+}
+
+// FetchUsage returns parsed usage windows for the given organization.
+func (c *Client) FetchUsage(ctx context.Context, orgID string) (*ParsedUsage, error) {
+	body, err := c.get(ctx, fmt.Sprintf("/organizations/%s/usage", orgID))
+	if err != nil {
+		return nil, err
+	}
+
+	var raw UsageResponse
+	if err := json.Unmarshal(body, &raw); err != nil {
+		return nil, fmt.Errorf("claudeai: parsing usage: %w", err)
+	}
+
+	return &ParsedUsage{
+		FiveHour:       parseWindow(raw.FiveHour),
+		SevenDay:       parseWindow(raw.SevenDay),
+		SevenDayOpus:   parseWindow(raw.SevenDayOpus),
+		SevenDaySonnet: parseWindow(raw.SevenDaySonnet),
+	}, nil
+}
+
+// FetchOverageLimit returns overage spend limit data for the given organization.
+func (c *Client) FetchOverageLimit(ctx context.Context, orgID string) (*OverageLimit, error) {
+	body, err := c.get(ctx, fmt.Sprintf("/organizations/%s/overage_spend_limit", orgID))
+	if err != nil {
+		return nil, err
+	}
+
+	var ol OverageLimit
+	if err := json.Unmarshal(body, &ol); err != nil {
+		return nil, fmt.Errorf("claudeai: parsing overage limit: %w", err)
+	}
+	return &ol, nil
+}
+
+// get performs an authenticated GET request and returns the response body.
+func (c *Client) get(ctx context.Context, path string) ([]byte, error) {
+	ctx, cancel := context.WithTimeout(ctx, requestTimeout)
+	defer cancel()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, baseURL+path, nil)
+	if err != nil {
+		return nil, fmt.Errorf("claudeai: creating request: %w", err)
+	}
+
+	req.Header.Set("Cookie", "sessionKey="+c.sessionKey)
+	req.Header.Set("Accept", "application/json")
+	req.Header.Set("User-Agent", "cburn/1.0")
+
+	//nolint:gosec // URL is constructed from const baseURL
+	resp, err := c.http.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("claudeai: request failed: %w", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	switch resp.StatusCode {
+	case http.StatusUnauthorized, http.StatusForbidden:
+		return nil, ErrUnauthorized
+	case http.StatusTooManyRequests:
+		return nil, ErrRateLimited
+	}
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return nil, fmt.Errorf("claudeai: unexpected status %d", resp.StatusCode)
+	}
+
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxBodySize))
+	if err != nil {
+		return nil, fmt.Errorf("claudeai: reading response: %w", err)
+	}
+	return body, nil
+}
+
+// parseWindow converts a raw UsageWindow into a normalized ParsedWindow.
+// Returns nil if the input is nil or unparseable.
+func parseWindow(w *UsageWindow) *ParsedWindow {
+	if w == nil {
+		return nil
+	}
+
+	pct, ok := parseUtilization(w.Utilization)
+	if !ok {
+		return nil
+	}
+
+	pw := &ParsedWindow{Pct: pct}
+
+	if w.ResetsAt != nil {
+		if t, err := time.Parse(time.RFC3339, *w.ResetsAt); err == nil {
+			pw.ResetsAt = t
+		}
+	}
+
+	return pw
+}
+
+// parseUtilization defensively parses the polymorphic utilization field.
+// Handles int (75), float (0.75 or 75.0), and string ("75%" or "0.75").
+// Returns value normalized to 0.0-1.0 range.
+func parseUtilization(raw json.RawMessage) (float64, bool) {
+	if len(raw) == 0 {
+		return 0, false
+	}
+
+	// Try number first (covers both int and float JSON)
+	var f float64
+	if err := json.Unmarshal(raw, &f); err == nil {
+		return normalizeUtilization(f), true
+	}
+
+	// Try string
+	var s string
+	if err := json.Unmarshal(raw, &s); err == nil {
+		s = strings.TrimSuffix(strings.TrimSpace(s), "%")
+		if v, err := strconv.ParseFloat(s, 64); err == nil {
+			return normalizeUtilization(v), true
+		}
+	}
+
+	return 0, false
+}
+
+// normalizeUtilization converts a value to 0.0-1.0 range.
+// Values > 1.0 are assumed to be percentages (0-100 scale).
+func normalizeUtilization(v float64) float64 {
+	if v > 1.0 {
+		return v / 100.0
+	}
+	return v
+}

internal/claudeai/types.go

@@ -0,0 +1,59 @@
+package claudeai
+
+import (
+	"encoding/json"
+	"time"
+)
+
+// Organization represents a claude.ai organization.
+type Organization struct {
+	UUID         string   `json:"uuid"`
+	Name         string   `json:"name"`
+	Capabilities []string `json:"capabilities"`
+}
+
+// UsageResponse is the raw API response from the usage endpoint.
+type UsageResponse struct {
+	FiveHour       *UsageWindow `json:"five_hour"`
+	SevenDay       *UsageWindow `json:"seven_day"`
+	SevenDayOpus   *UsageWindow `json:"seven_day_opus"`
+	SevenDaySonnet *UsageWindow `json:"seven_day_sonnet"`
+}
+
+// UsageWindow is a single rate-limit window from the API.
+// Utilization can be int, float, or string — kept as raw JSON for defensive parsing.
+type UsageWindow struct {
+	Utilization json.RawMessage `json:"utilization"`
+	ResetsAt    *string         `json:"resets_at"`
+}
+
+// OverageLimit is the raw API response from the overage spend limit endpoint.
+type OverageLimit struct {
+	IsEnabled          bool    `json:"isEnabled"`
+	UsedCredits        float64 `json:"usedCredits"`
+	MonthlyCreditLimit float64 `json:"monthlyCreditLimit"`
+	Currency           string  `json:"currency"`
+}
+
+// SubscriptionData is the parsed, TUI-ready aggregate of all claude.ai API data.
+type SubscriptionData struct {
+	Org       Organization
+	Usage     *ParsedUsage
+	Overage   *OverageLimit
+	FetchedAt time.Time
+	Error     error
+}
+
+// ParsedUsage holds normalized usage windows.
+type ParsedUsage struct {
+	FiveHour       *ParsedWindow
+	SevenDay       *ParsedWindow
+	SevenDayOpus   *ParsedWindow
+	SevenDaySonnet *ParsedWindow
+}
+
+// ParsedWindow is a single rate-limit window, normalized for display.
+type ParsedWindow struct {
+	Pct      float64 // 0.0-1.0
+	ResetsAt time.Time
+}