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/config/config.go

@@ -1,12 +1,9 @@
 package config
 
 import (
-	"encoding/json"
 	"fmt"
 	"os"
-	"time"
 
-	"github.com/fujin/anthropic-proxy/internal/auth"
 	"gopkg.in/yaml.v3"
 )
 
@@ -67,15 +64,6 @@ type LoggingConfig struct {
 	Compress   bool   `yaml:"compress"`
 }
 
-type claudeCredentialsJSON struct {
-	ClaudeAiOauth struct {
-		AccessToken      string `json:"accessToken"`
-		RefreshToken     string `json:"refreshToken"`
-		ExpiresAt        int64  `json:"expiresAt"`
-		SubscriptionType string `json:"subscriptionType"`
-	} `json:"claudeAiOauth"`
-}
-
 func Load(path string) (*Config, error) {
 	data, err := os.ReadFile(path)
 	if err != nil {
@@ -128,47 +116,3 @@ func Load(path string) (*Config, error) {
 
 	return cfg, nil
 }
-
-func DefaultCredentialPath() string {
-	home, err := os.UserHomeDir()
-	if err != nil {
-		return ""
-	}
-	return home + "/.claude/.credentials.json"
-}
-
-func LoadDefaultCredentials() ([]*auth.Credential, error) {
-	path := DefaultCredentialPath()
-	if path == "" {
-		return nil, nil
-	}
-
-	data, err := os.ReadFile(path)
-	if err != nil {
-		if os.IsNotExist(err) {
-			return nil, nil
-		}
-		return nil, err
-	}
-
-	var cf claudeCredentialsJSON
-	if err := json.Unmarshal(data, &cf); err != nil {
-		return nil, err
-	}
-
-	oauth := cf.ClaudeAiOauth
-	if oauth.AccessToken == "" {
-		return nil, fmt.Errorf("no access token in %s", path)
-	}
-
-	cred := &auth.Credential{
-		ID:           "claude-native",
-		Email:        oauth.SubscriptionType,
-		AccessToken:  oauth.AccessToken,
-		RefreshToken: oauth.RefreshToken,
-		ExpiresAt:    time.UnixMilli(oauth.ExpiresAt),
-		FilePath:     path,
-	}
-
-	return []*auth.Credential{cred}, nil
-}