feat(demo): add builder gallery

[sumfxn]

Summary

Builder Gallery V1 for apps/demo:

• replaces the old single-widget home with a shadcn-style USDH builder gallery
• shows SDK surfaces for Tracks 1-4: discovery, outcomes, USDH orders, useful flows
• adds a server-side read-only mainnet market snapshot with mocked fallback
• keeps wallet connection secondary; the page is useful without connecting
• updates root build/typecheck so CI validates the demo app too

Safety

• No SDK public API changes.
• No widget public API changes.
• No /exchange calls for gallery data.
• Live Hyperliquid reads are request-time only, have a global fallback deadline, and never block next build.
• Widget package output is patch-only for React dev dependency hygiene; release remains gated separately.
• Track 5 HyperEVM direct swap is not presented as shipped.

Validation

• pnpm install --frozen-lockfile --ignore-scripts
• pnpm lint
• pnpm build
• pnpm typecheck
• pnpm test
• git diff --check
• pnpm changeset status --since=origin/main
• Chrome/CDP screenshots and overflow audit for desktop + mobile

[sumfxn]

Superseded by the clean main-based stack merged through #58-#62. Closing this draft backup to keep the public PR list clean.