refactor(server side): split index.ts into focused modules

[diplexion]

Hey deyo i really interesting of ur project cuz i need something like for testing my servers but ngrok is ass 🗡️ so and i decided help u with develop and make this refactoring <3

Result of refactoring

Why

server/src/index.ts was 635 lines and mixed several unrelated concerns:
routing, subdomain/host utilities, the binary WS envelope protocol, the
TunnelDO durable object (the actual tunnel logic), and ~250 lines of
inline HTML templates. This made the core tunnel logic hard to find and
hard to unit-test in isolation from Cloudflare-specific types.

What changed

Pure code move, no behavior change:

• types.ts — Env, PendingRequest, ProxyResponse
• subdomain.ts — host/subdomain parsing, sanitization, sharding
• envelope.ts — binary request/response envelope (de)serialization
• tunnel-do.ts — TunnelDO durable object (WS lifecycle, proxying, auth)
• html/root.ts / html/offline.ts — inline HTML templates
• index.ts — thin fetch handler / routing only (~60 lines)

Testing

• npx tsc --noEmit passes with no errors
• npx wrangler deploy --dry-run bundles successfully and resolves the
  TunnelDO durable object binding correctly