Integrate beat maker with pack pages and fix marketplace UX gotchas

Commit notes

Sequencer <-> storefront integration: - Pack pages get a "Try these sounds in the beat maker" card linking to /studio?creator=<slug>&pack=<slug> - The studio loads a published pack's preview samples (up to 8) as sequencer tracks via the public preview endpoint, so guests can jam with a pack before buying, and shows a dismissible banner linking back to the pack with a buy CTA

UX fixes: - Add /auth/signup and /auth/login pages (email/password via existing backend endpoints plus Google), fixing three checkout-flow links that 404'd; guest emails are carried over via ?email= so purchases link up - Replace two dead /settings/subscription upgrade CTAs with buttons that open the global subscription drawer (drawer state moved into the subscription store) - Fix /dashboard bouncing signed-in users to a bogus /auth/callback URL on hard refresh: wait for the auth check, show an inline sign-in prompt when logged out - Point the reset-password success CTA at the new sign-in page - Make the header logo link home; add a sign-in button to the library's signed-out state - Allow http/ws localhost in the CSP during dev only: the enforced https:-only connect-src blocked every browser call to the local backend, breaking the app in local development

Co-Authored-By: Claude Fable 5 <[email redacted]> Claude-Session: [private session redacted]