[Phantomcall]

Fixes two frontend flows that called HTTP endpoints which do not exist. Both
touch the same wallet/Soroban path, so they ship together.

#1755 — Dispute page used nonexistent endpoints and a stub sign

/api/v1/blockchain/markets/{id}/disputes is not served by
services/api/openapi.yaml, and useTransaction's sign just returned
{ marketId, reason } — no transaction was built, the reason was never a
contract argument, and write errors were described with the 'resolve'
context.

• Dispute status is now read from api.getBlockchainMarket, the endpoint
  the backend actually serves; the disputed-outcome list is optional on that
  view and renders "no dispute filed yet" when absent.
• Filing now builds and signs the contract's real file_dispute entry point
  and submits it to Soroban RPC.
• Write errors are described with the new 'dispute' action context.

#1756 — Claim page used a nonexistent endpoint

/api/v1/blockchain/bets/{id} and /api/v1/blockchain/markets/{id}/claim are
not served either.

• The bet is loaded from the typed client's getUserBets response for the
  connected wallet (the endpoint the backend does serve).
• Claiming now invokes the contract's claim_winnings entry point via the
  wallet.
• Write errors use the new 'claim' action context.

Shared: lib/wallet/soroban.ts

There is no Soroban client in the frontend, and no contract id / RPC env vars,
so this adds a small, tested client used by both pages: build → wallet-sign →
submit. It reads NEXT_PUBLIC_CONTRACT_ID / NEXT_PUBLIC_SOROBAN_RPC_URL /
NEXT_PUBLIC_STELLAR_NETWORK_PASSPHRASE. When the contract id or RPC URL is
unset, the pages render a clear "not configured" message and disable the
submit button instead of calling a nonexistent endpoint — so a deployment
that has not wired these yet degrades instead of breaking.

The existing lib/wallet/balance.ts already imported @stellar/stellar-sdk
without declaring it (it is absent from package.json and the lockfile), which
would fail a real next build. This PR declares the dependency
(^12.3.0) and adds it to package-lock.json, fixing that too.

Other changes

• WriteAction gains 'dispute' and 'claim' with their 101 (NotAuthorized)
  messages.
• Docs: docs/data-flow.md documents the client-side contract-write path;
  frontend/README.md and frontend/.env.example document the new env vars.
• Tests: src/lib/wallet/__tests__/soroban.test.ts covers config resolution,
  argument encoding (verified against the real SDK), build/sign/submit wiring,
  the send/on-chain failure paths, and the unconfigured fast-path.

Verification

• soroban.ts was exercised end-to-end against the real @stellar/stellar-sdk
  at both 12.3.0 and 17.0.1 (rpc namespace, valid 1-op transaction, hash
  returned, unconfigured path throws).
• Changed files pass prettier --check with the repo's config.

Note on CI

npm ci fails on main today with a pre-existing peer-dependency conflict
(openapi-typescript@7 wants typescript@^5.x); this is unrelated to this PR
and reproduces on unmodified main. The lockfile/package.json pair added
here resolves correctly (npm ci --legacy-peer-deps --dry-run installs
@stellar/stellar-sdk@12.3.0).

Closes #1755
Closes #1756

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