Add Twitch account linking UI

[cmyui]

Summary

• add a Twitch link kickoff route alongside Discord linking
• show linked Twitch account state in settings connections
• add an unlink action that calls the Akatsuki API

Testing

• gofmt -w app/handlers/connections/discord.go app/states/settings/settings.go main.go
• go test ./app/handlers/connections ./app/states/settings -run TestNonExistent