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/version/version.go

@@ -0,0 +1,8 @@
+// Package version provides the fallback Claude Code client version used when
+// no sniffed profile is available. This constant is shared between the upstream
+// proxy client and the rate limit usage poller.
+package version
+
+// ClaudeCodeFallback is the Claude Code CLI version string used as a fallback
+// when no real version is obtained from sniffing.
+const ClaudeCodeFallback = "2.1.92"