feat: add cookie support for requests and responses

[slverma]

Summary

• Request Cookies tab — new "Cookies" tab on every request lets users define name/value cookie pairs that are serialized into a Cookie header when sending. Supports {{variable}} interpolation, per-cookie enable/disable toggles, and persists with the saved request.
• Response Cookies tab — Set-Cookie headers from responses are parsed into structured objects and displayed in a new "Cookies" tab on the response panel, showing name, value, Path, HttpOnly, and Secure attributes.
• Extension host — parseSetCookie helper correctly handles multi-value Set-Cookie arrays (which axios preserves as string[]), splitting on the first = to handle values that contain =. Existing explicit Cookie headers are never overridden.

Test Plan

• Add one or more cookies to a request, send it — verify the Cookie header appears in the request (inspect via a tool like httpbin.org/cookies)
• Disable a cookie via its checkbox — verify it is excluded from the sent Cookie header
• Use {{variable}} syntax in a cookie name or value — verify interpolation works
• Hit an endpoint that sets Set-Cookie headers — verify the Cookies tab appears on the response with the correct name/value/attributes
• Verify cookies survive panel close and reopen (persistence)
• Verify a manually-set Cookie request header takes priority over the Cookies tab (no override)