Add unit tests for stellar.ts; fix build-breaking errors across the codebase

[cokehill]

What changed

1. Add test coverage for src/lib/stellar.ts

package.json already had "test": "vitest run" wired up, but there
was no vitest.config.ts and zero test files anywhere in the repo.
Added:

• vitest.config.ts — points vitest at src/**/*.test.ts
• src/lib/stellar.test.ts — 21 unit tests covering the pure,
  network-free helpers (getNetworkPassphrase, getHorizonUrl,
  parseAsset, isValidPublicKey, isValidSecretKey), including the
  public/secret-key-swap case that's an easy mistake for consumers of
  this library to make.

2. Fix npm run build (was failing before this PR)

Running the build surfaced a series of pre-existing, unrelated issues:

• src/index.ts — raw CSS had been appended after the exports,
  breaking esbuild's parser. Moved the intended style into
  src/styles/index.css.
• src/components/ui/Field.tsx — a duplicate, unreachable
  {error && (...)} block was pasted inside return (, before the
  root element (invalid JSX). Removed it. Also fixed two type
  mismatches: value passed to AssetInput needed the real Asset
  type instead of a loose shape, and error (typed string | null)
  needed ?? undefined before being passed to AssetInput.
• src/components/TransactionBuilder/OperationsStep.tsx — removed an
  orphaned validation snippet referencing undefined variables, and
  fixed a Type 'unknown' is not assignable to type 'ReactNode' error
  by wrapping op.params.destination in Boolean(...).
• src/lib/stellar.ts — removed an unused TransactionParams import,
  and fixed Memo.return()'s type mismatch (its types only accept
  string, unlike Memo.hash()) by passing the hex string directly
  instead of Buffer.from(...).
• src/lib/operationDefinitions.ts — two validate functions used
  !v || v.trim(), which TypeScript narrows unknown to {} rather
  than string on the truthy branch (a known TS quirk). Switched to
  typeof v !== "string" || ..., which narrows correctly.
• Removed unused import React from "react" across several
  components (project uses "jsx": "react-jsx"), restoring named
  imports (useEffect, Fragment) where a file actually needed them.

Testing

• npx vitest run — 21/21 passing
• npm run build — CJS, ESM, and DTS all build successfully