feat: improve logger with self-contained log lines

[Thanaen]

Closes #95

Summary

• Each log line now includes the HTTP method, URL, status code, and request duration so concurrent requests can be distinguished at a glance
• Adds a logFormat: "legacy" option to preserve the previous log format for backward compatibility
• Documents the new logFormat option in the logger docs

Before

ℹ Request being sent to: https://website.com/api/users
ℹ Request being sent to: https://website.com/api/posts
✓ Request succeeded { ... }         ← which one?
✗ Request failed with status: 401   ← which one?

After

ℹ [GET] https://website.com/api/users
ℹ [GET] https://website.com/api/posts
✓ [GET] https://website.com/api/users — 200 OK (145ms)
✗ [POST] https://website.com/api/posts — 401 Unauthorized (89ms)

Test plan

• pnpm build in packages/logger — ESM/CJS bundles succeed
• Manual test: create a fetch client with logger(), fire 2+ parallel requests, verify each log line is self-contained
• Manual test: use logFormat: "legacy" and confirm old log messages are unchanged
• Verify verbose: true still logs response bodies on success and error

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