feat(transaction-panel): send assetIssuer and show a balance hint

[16navigabraham]

Summary

Closes #565. Closes #566 (duplicate of #565 — same title/problem, filed separately).

TransactionPanel's asset selector already let a user pick a non-native asset from their balances (added in a prior PR), but the actual submission never sent enough information for the backend to know which issuer's asset to pay with, and the amount input gave no feedback on how much of the selected asset was actually available.

Problem

• TransactionParams (src/lib/client.ts) has no assetIssuer field, and TransactionPanel's submitTransaction() only ever sent asset: selectedAsset (an asset code like "USDC") — never the issuer account. Two different tokens can share the same code with different issuers, so this payload is ambiguous for anything other than native XLM.
• The Amount input showed the selected asset's balance only as part of an error message once you'd already typed an amount that exceeded it — there was no passive hint showing what's available before you type.

What changed

• src/lib/client.ts — added assetIssuer?: string to TransactionParams.
• src/components/TransactionPanel.tsx
  • submitTransaction() now passes assetIssuer: selectedAssetBalance?.assetIssuer in the transaction.submit(...) call — undefined for native XLM (which has no issuer), and the balance's assetIssuer for anything else.
  • The Amount input now shows Balance: {balance} {asset} via the existing Input component's hint prop, which is automatically suppressed whenever a validation error is showing (no double messaging).
• src/components/TransactionPanel.test.tsx — 3 new tests: assetIssuer is included for a non-native asset and correctly matches the selected balance's issuer, assetIssuer is undefined for XLM, and the balance hint updates when the asset selection changes.

Acceptance criteria

• A dropdown shows all assets from the connected wallet's balance list (already implemented before this PR)
• The selected asset code is reflected in the amount input label (already implemented before this PR)
• The transaction payload includes the correct assetCode and assetIssuer
• XLM (native) is the default selected asset (already implemented before this PR)
• The balance for the selected asset is shown as a hint near the amount input
• Existing amount validation and the Send button disabled state still work (unchanged, verified via the existing test suite)

Testing

• npx vitest run src/components/TransactionPanel.test.tsx → same 25/36 pass as main plus the 3 new tests (28 total passing here); the 11 failures are pre-existing and unrelated — see note below.
• npm run typecheck → clean.

Note for maintainers

While working on this I found main already has 11 pre-existing, unrelated failures in TransactionPanel.test.tsx (confirmed via git stash against a clean checkout) — every test that completes a submission through the confirmation modal fails with "Wallet not connected", because the shared useSorokit() mock at the top of the file was never updated to include a client field after the "context client" refactor (client now comes from useSorokit(), not a direct getClient() call). My 3 new tests work around it locally (client: getClient() in their own mock), but I left the shared, already-broken tests alone since fixing them isn't in scope for this issue. Flagging in case it's worth a follow-up.

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