feat(stellar-kit): implement transaction readiness pipeline and web U…

[samuelfrancis163-eng]

Closes #50

• References the original issue with Closes #NNN in the PR body.
• Acceptance criteria on the issue are checked off individually in the PR description.
• pnpm lint passes locally.
• pnpm typecheck passes locally.
• pnpm test passes locally.
• pnpm contract:test passes locally (if anything under contracts/ changed).
• pnpm format:check passes or pnpm format was applied.

Stellar correctness

• Any new Stellar public-key / secret-key parsing matches the branded types.
• Amounts remain 7-decimal strings (not number / floats) through the change.
• Memo rules are enforced if touched.
• Horizon error handling still maps to the typed StellarKitError codes correctly.

Security impact

• R0: No new console.log / logger / error .message contains a raw secret.
• R1: No secret echoed in UI text, copy-to-clipboard, or URL.
• R2: No new localStorage / cookie / persistent store of secrets.
• R3: No secret in a constructed URL, callback, query param, or auth header.
• R4: Structural validation runs before SDK calls that could leak into stack traces.
• R5: All new secret-accepting APIs accept unknown, Zod-validate, and return branded types.
• R6: No committed real secret keys anywhere in fixtures/examples.

Tests

• New public functions in packages/ have a positive and a negative Vitest case.
• Any new branch in error mapping or status transitions has a test.
• For contract changes: happy path + error path tests added under src/test.rs covering the specific new guard.
• Tests do not print or assert on raw secret keys.

Documentation updates

• /docs/_____.md updated for any new user-facing package export.
• README or /docs/LOCAL_SETUP.md updated if setup steps changed.

No secret leakage + no mainnet risk

• Diff search for S-prefixed 56-char secrets shows only synthetic throwaway fixtures.
• Defaults remain testnet-first; any new mainnet path is behind assertNetworkAllowed(...).

Risk / follow-ups

Any out-of-scope follow-up issues or known follow-up work. List them here and (if already created) link the follow-up issue numbers.

[El-swaggerito]

\nThis PR is currently blocked by merge conflicts.\n\nPlease update the branch with the latest main branch and resolve the conflicts before it can be merged.

[El-swaggerito]

\nGitHub has not finished calculating whether this PR can be merged cleanly.\n\nThe auto-merge automation will skip this PR for now. Re-run the automation later.