feat: stellar configuration panel

[starkiVector-090]

What was implemented

New files in apps/frontend/src/components/app/stellar/:

• AssetPairEditor.tsx — inline form to add/remove asset pairs with base/counter asset
  type, code, and issuer fields
• ContractAddressInputs.tsx — add/remove Soroban contract addresses with inline
  validation against the existing validateContractAddress utility
• SorobanRpcInput.tsx — URL input with async connectivity check button and status
  feedback (mirrors HorizonUrlInput)
• StellarConfigPanel.tsx — main panel composing all four field groups into a form with
  save/reset actions
• index.ts — barrel export for the whole stellar directory

Updated files:

• useStellarConfigForm.ts — added sorobanConnectivityStatus, sorobanConnectivityResult,
  and checkSorobanConnectivity to the hook and its return type; also resets Soroban
  connectivity state when sorobanRpcUrl changes
• vitest.config.ts (frontend) — added @craft/stellar and @craft/types source aliases so
  tests resolve without a build step
• stellar.test.tsx — 44 tests covering all new components and the updated hook

Edge cases handled:

• Auto-updating horizonUrl when network switches only if the URL still matches the old
  default
• Network/Horizon URL mismatch validation (testnet URL on mainnet config and vice
• Duplicate contract name detection in ContractAddressInputs
• Connectivity state reset on URL change for both Horizon and Soroban RPC

closes #19

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