refactor: modularize codebase — deduplicate, extract, clean up

- Unify duplicate uTLS transports into shared internal/transport package
- Extract shared version constant into internal/version
- Move LoadDefaultCredentials from config to auth (remove config→auth import)
- Deduplicate handler.go: extract telemetry/error helpers (324→268 lines)
- Break up main.go::run() into initCredential/initEmbedded
- Eliminate logging.Config duplication (use config.LoggingConfig directly)
- Extract logWriter to embedded/log.go, SSE fixtures to consts in sniff.go
- Use uTLS client for usage polling (consistent TLS fingerprint)
- Handle sjson.SetBytes errors in sanitize.go instead of silently swallowing
- Document reverse-engineered magic values in billing.go
- Unexport Credential.CooldownUntil (internal state)
- Replace hardcoded auth bypass paths with map in server.go

internal/ratelimit/usage.go

@@ -7,8 +7,13 @@ import (
 	"io"
 	"net/http"
 	"time"
+
+	"github.com/fujin/anthropic-proxy/internal/transport"
+	"github.com/fujin/anthropic-proxy/internal/version"
 )
 
+var usageClient = transport.NewHTTPClient(10 * time.Second)
+
 const usageURL = "https://api.anthropic.com/api/oauth/usage"
 
 type RateLimit struct {
@@ -17,17 +22,17 @@ type RateLimit struct {
 }
 
 type ExtraUsage struct {
-	IsEnabled   bool     `json:"is_enabled"`
+	IsEnabled    bool     `json:"is_enabled"`
 	MonthlyLimit *float64 `json:"monthly_limit"`
 	UsedCredits  *float64 `json:"used_credits"`
 	Utilization  *float64 `json:"utilization"`
 }
 
 type UsageResponse struct {
-	FiveHour     *RateLimit  `json:"five_hour"`
-	SevenDay     *RateLimit  `json:"seven_day"`
-	SevenDaySonnet *RateLimit `json:"seven_day_sonnet"`
-	ExtraUsage   *ExtraUsage `json:"extra_usage"`
+	FiveHour       *RateLimit  `json:"five_hour"`
+	SevenDay       *RateLimit  `json:"seven_day"`
+	SevenDaySonnet *RateLimit  `json:"seven_day_sonnet"`
+	ExtraUsage     *ExtraUsage `json:"extra_usage"`
 }
 
 func fetchUsage(ctx context.Context, token string) (*UsageResponse, error) {
@@ -41,9 +46,9 @@ func fetchUsage(ctx context.Context, token string) (*UsageResponse, error) {
 	req.Header.Set("Authorization", "Bearer "+token)
 	req.Header.Set("Content-Type", "application/json")
 	req.Header.Set("anthropic-beta", "oauth-2025-04-20")
-	req.Header.Set("User-Agent", "claude-cli/2.1.92")
+	req.Header.Set("User-Agent", "claude-cli/"+version.ClaudeCodeFallback)
 
-	resp, err := http.DefaultClient.Do(req)
+	resp, err := usageClient.Do(req)
 	if err != nil {
 		return nil, fmt.Errorf("request: %w", err)
 	}