fix(widget): polish swap quote flow

[sumfxn]

What

Polishes the packaged swap widget quote flow, balance handling, action button states, and related widget tests.

Why

Improves the production widget path while keeping the widget narrowly scoped to the USDH swap experience.

Tests

• npx pnpm@10.33.2 verify
• git diff --check
• Changeset added (or N/A): N/A, no release

Notes

• No widget API expansion.
• No registry UI is exported from the widget package.