Reposition Grainstash around audio-native catalog + storefront

Commit notes

Implements the new product positioning: lead with hosting, organizing, and selling sounds (audio-native catalog + creator storefront), with the step sequencer demoted to a free funnel/preview feature.

Landing & routing: - New marketing landing page at / (hero, value props, pricing) that adapts CTAs to auth state - Move the step sequencer from / to /studio (the /sequencer/* path is a backend asset proxy, so /studio avoids a route conflict) - Update Header nav, CommandPalette, and labels to point at /studio

Fee model (fixes adverse selection): - Add PRO_FEE_PERCENT config (default 3%); Pro/Team sellers now pay a low 3% fee instead of 0%, keeping platform incentives aligned with high-volume sellers while still undercutting Gumroad (~13%) / Beatstars (12%). Free tier unchanged at 15%. - Drive dashboard fee percentages from config; expose free/pro fee percents for accurate upsell copy - Update SubscriptionDrawer and dashboard messaging from "keep 100% / 0%" to the new 3% framing

Docs: - Rewrite README and CLAUDE.md product overview around the new positioning; note the docs/README that predate the Stripe migration

[private session redacted]