feat(core,react): add the blockchain helper client

[TaprootFreak]

Step 5 of #198 (the optional one): move the blockchain helper endpoints into the packages, so any
wallet-integrated consumer can read balances and build transfers without a local wrapper.

Changes

@dfx.swiss/core

• BlockchainUrl (blockchain/balances, blockchain/transaction, blockchain/broadcast)
• GetBlockchainBalances, BlockchainBalance, BlockchainBalances, CreateBlockchainTransaction,
  UnsignedTransaction, BroadcastTransaction, BroadcastResult
• BlockchainApi, wired into DfxApiClient as client.blockchain

@dfx.swiss/react

• useBlockchain() with getBalances, createTransaction, broadcastTransaction
• Re-exports of the new types

Tests cover all three calls, including the request without an asset filter.

Notes

• No chain-specific libraries. createTransaction returns the raw transaction and its encoding
  (base64 for Solana, hex elsewhere); decoding and signing it needs a chain library, so it stays with
  the caller. Pulling one in here would put a wallet dependency into every consumer of the SDK.
• Balances come back as asset ids, not assets, and an asset the address holds nothing of can be
  missing from the response entirely — so the response is returned as the API sends it rather than
  being pre-joined against an asset list the SDK does not have.
• All three endpoints are rate-limited on the API side and require a session.

Additive only — no existing export changes shape.

Version fields, changelogs and lockfile pins are untouched, per CONTRIBUTING.

[TaprootFreak]

Took 2 review passes to reach a clean run.

Two things came out of the first pass:

• The comment on UnsignedTransaction claimed "base64 for Solana, hex elsewhere". For Tron the API
  actually returns JSON while still declaring encoding: 'hex', so the comment would have misled
  anyone who trusted it over checking. It now says only that the payload format is chain-specific and
  is not always what encoding says.
• blockchain was inserted between bank and bankAccount in the client list, which is otherwise
  alphabetical.

One reported point was not taken: CreateBlockchainTransaction.blockchain accepting the full
Blockchain enum while only Solana and Tron are served. That mirrors the API's own DTO, which
validates against the same unrestricted enum and rejects the rest at runtime — narrowing it here would
make the client disagree with the contract it describes.