Scaffold Aether: STRK20 integration layer and design system

Next.js 15 + TypeScript + Tailwind v4, pinned to the exact stack that
carries STRK20 support. `starknet@10.4.0` is the first release exposing
WalletAccountV6, strk20InvokeTransaction and STRK20_ACTION; the `latest`
line (10.0.2) lacks all three, so every STRK20 package is pinned exactly
rather than floated.

The private-DeFi action builder follows the four-action sequence shipped
in @avnu/avnu-sdk@4.2.0 (withdraw input, withdraw fee, open note, invoke)
rather than the abbreviated two-action example in the published docs,
which omits both withdrawals and produces a transaction the pool rejects.

assertNeverUnshields() enforces the product's core invariant before any
signature: a withdraw is permitted only to a helper contract taking part
in the same atomic transaction, so routing through DeFi cannot become an
unshield to a user-controlled address.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>