feat(studio): cascade verification UI

[ssilvius]

Summary

Sean: "the server has no UI for me to see changes." Confirmed -- App.tsx was a hardcoded mock ("primary-500" as literal text, dialog with no wiring). Nothing in it talked to the real registry or the studio API.

Replace with a real cascade verification surface:

• Three swatch grids: Accent (8 variants), Highlight (6 variants), Reference (background, foreground, primary, primary-foreground -- should not change)
• Each swatch renders via `var(--rafters-${name})`, so HMR pushes from the vite-plugin update them in place
• "Apply Easter palette" button fires `setToken(accent -> silver-true-honey-500)` then `setToken(highlight -> silver-true-sky-200)`
• StatusPill surfaces pending/ok/error states from the WebSocket round trip

This is the visual gate for the cascade fix in #1433 (this PR's base). With both merged, one setToken per family should cascade to every -hover/-active/-foreground/-border/-ring variant; the Reference section should stay put.

Test plan

• Pre-existing studio test suite was 45/149 failing before this change; unchanged after (failures are API-side, not UI)
• Biome clean
• Manual: run `pnpm --filter @rafters/studio dev` against this repo, click "Apply Easter palette," observe accent variants turn amber and highlight variants turn sky -- without page reload, with reference section unchanged

🤖 Generated with Claude Code