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
  (*:focus-visible) into src/styles/index.css.
• src/components/ui/Field.tsx — a duplicate, unreachable
  {error && (...)} block was pasted directly inside return (,
  before the root element (invalid JSX). Removed it — the correct
  {error && ...} rendering already exists later in the file. Also
  fixed two type mismatches: the value passed to AssetInput was
  cast to a loose shape instead of the real Asset type, and error
  (typed string | null) needed ?? undefined before being passed to
  AssetInput, which only accepts string | undefined.
• src/components/TransactionBuilder/OperationsStep.tsx — an orphaned
  field-validation snippet referenced undefined variables (fieldDef,
  newValue, fieldKey) and wasn't wired into any handler; removed
  it. Also fixed a Type 'unknown' is not assignable to type 'ReactNode' error by wrapping op.params.destination in
  Boolean(...) before the &&.
• src/lib/stellar.ts — removed an unused TransactionParams import,
  and fixed a real SDK type mismatch: Memo.return()'s type
  definition only accepts string (unlike Memo.hash(), which
  accepts string | Buffer), so passing Buffer.from(value, "hex")
  failed to typecheck. Confirmed against the SDK's runtime
  implementation that passing the hex string directly is equivalent.
• src/lib/operationDefinitions.ts — two validate functions
  (added for the payment operation's destination and amount
  fields) narrowed their unknown parameter with !v || ..., which
  TypeScript narrows 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" (and one unused
  XdrImport import) across several components — the project uses
  "jsx": "react-jsx", so the default React import isn't needed.
  Restored named imports (useEffect, Fragment) where a file
  actually used React.something.

Testing

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