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/proxy/billing.go

@@ -11,9 +11,13 @@ import (
 	"github.com/tidwall/sjson"
 )
 
+// fingerprintSalt is the fixed salt used by Claude Code for billing header
+// fingerprint computation. Extracted from the Claude Code CLI source.
 const fingerprintSalt = "59cf53e54c78"
 
 func computeFingerprint(firstUserMessage string, version string) string {
+	// UTF-16 character indices sampled from the first user message, matching
+	// the Claude Code CLI's fingerprinting algorithm.
 	indices := []int{4, 7, 20}
 	runes := utf16.Encode([]rune(firstUserMessage))
 	var chars string