add track sharing and deployment config
- implement ShareButton component with copy-to-clipboard
- add /track/[id] route for shareable track links
- hide portal link when already on portal page
- configure wrangler.toml for cloudflare pages with nodejs_compat flag
- add deployment commands to justfile (deploy-frontend, deploy-backend)
- improve auth to support Bearer token for cross-domain requests
- fix accessibility warnings in TrackItem component
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Co-Authored-By: Claude <noreply@anthropic.com>
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