feat: allow 'anakin' handler in domain config handler chain editor

[MayurK-cmd]

What does this PR do?

Fixes #46
Adds the anakin handler to the available options in the Domain Configs
handler-chain editor in the web dashboard.

The server already registers an anakin handler (anakin.io API fallback)
whenever ANAKIN_API_KEY is set (server/cmd/server/main.go:83-84, handler
name "anakin" in server/internal/handler/api.go:92), and a domain config's
handlerChain is passed through as the chain's AllowedHandlers filter. The
form previously hardcoded the available handlers to ['http', 'browser'], so
anakin could not be selected from the UI.

How to test

1. Start the server with ANAKIN_API_KEY set so the anakin handler is enabled.
2. Open the web dashboard → Domain Configs → Add Config.
3. In the Handler Chain section, confirm a + anakin button now appears
   (in addition to + http / + browser) and can be added and reordered.
4. Save a config with handlerChain including anakin and confirm it persists
   (GET /v1/domain-configs returns the chain as saved).

Checklist

• Tests pass (cd server && go test ./...) — no Go code changed
• No breaking changes to existing API endpoints
• Documentation updated — N/A