feat: relay account support with multi-format direct proxy

[icebear0828]

Summary

Closes #105

• Relay accounts: support third-party relay services via API Key + Base URL, with type: "native" | "relay" discriminator on AccountEntry
• Format-aware direct proxy: relay accounts can specify format (codex/openai/anthropic/gemini). Non-codex formats bypass Codex translation — requests forward directly to the relay, responses pipe back as-is
• Account pool integration: relay accounts participate in the same rotation pool, with allowedModels filtering and format-preference in acquire()
• Dashboard UI: Relay button in header, AddRelayAccount form (label, baseUrl, apiKey, format dropdown, allowedModels), grouped account list (ChatGPT / Relay sections), format badge on AccountCard

Key changes

Area	Files	What
Data model	src/auth/types.ts	AccountType, RelayFormat, new fields on Entry/Info/Acquired
Account pool	src/auth/account-pool.ts	addRelayAccount(), format-aware acquire(), JWT/quota skip, backfill
Codex API client	src/proxy/codex-api.ts	baseUrlOverride, relay mode (simple Bearer, no fingerprint)
Direct proxy	src/routes/shared/direct-proxy.ts	New — stream pipe / simplePost for non-codex relays
Route handlers	chat.ts, messages.ts, gemini.ts	Try format-matching relay before Codex translation
Schedulers	refresh-scheduler.ts, usage-refresher.ts	Skip relay accounts
Frontend	AddRelayAccount.tsx, AccountCard.tsx, AccountList.tsx, Header.tsx	Relay form, grouped list, format display

Test plan

• 45 new unit tests across 5 files (903 total, all passing)
• E2E: add openai-format relay → /v1/chat/completions routes to relay (verified via httpbin echo)
• E2E: delete relay → same request falls back to native Codex translation
• E2E: codex-format relay does NOT intercept OpenAI route (stays on Codex path)
• E2E: backfill — existing accounts get type=native, format=null after restart