Add dynamic label selection when creating a new issue:
- New GET /issues/new/labels endpoint returns label checkboxes for a repo
- When a repo is selected, labels are fetched via HTMX and displayed as
checkboxes with colored label badges
- CreateIssue handler now parses label_ids from form and passes them to
the Gitea API
- Shows "No labels available" message when repo has no labels
Closesleeworks-agents/gitea-mobile#67
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>