Remove redundant second slug UPDATE in createSet

[chiptus]

Problem

createSet in src/hooks/queries/sets/useCreateSet.ts performs a redundant second write:

1. The INSERT already includes slug: generateSlug(setData.name).
2. Immediately after, the function runs a second UPDATE that sets slug = generateSlug(data.name) and writes it back.

generateSlug (src/lib/slug.ts) only depends on its input string — it never uses the row ID. Since data.name === setData.name, the follow-up UPDATE writes the identical value. It's a wasted round-trip, and the "Generate and update the slug using the created ID" comment is stale and misleading.

createArtist already does this correctly: a single insert with the slug, no follow-up update.

Suggested fix

Remove the post-insert slug UPDATE block in createSet and return data directly (with artists: [], votes: []). Drop the misleading comment.

Context

Flagged by Copilot review on PR #34. Tracking separately rather than expanding the db-sync PR scope.