Wire the kudos button to the backend

Problem

The feed kudos button has no @click handler, so kudos can't be given from the feed. The Kudos model exists but no endpoint is wired and tb.toggleKudos only mutates the local array (and the activity kudos_count denormalized counter is never recomputed).

Acceptance

• Kudos give/remove endpoint (with the dedupe constraint from the data-integrity issue).
• Wire the feed + detail kudos buttons; optimistic UI; recompute activities.kudos_count.

Done: KudosToggleAction (POST /api/activities/{id}/kudos), game-api.toggleKudos, feed button wired with optimistic UI. Commit dc71d02.