feat: design stellar configuration

[lyon-1050]

Here's a summary of what was implemented for issue-018:

Files created:

1. apps/frontend/src/lib/customization/validate-stellar.ts — Zod
   schema (stellarConfigSchema) and validateStellarConfig() function
   covering:
   - Network selection (mainnet | testnet) with mismatch detection
   against Horizon URL

• Horizon URL validation (format + network/URL consistency check)
  • Soroban RPC URL (optional, URL format)
  • Asset pairs: type-aware validation (alphanum4 max 4 chars,
    non-native requires valid G… issuer, max 20 pairs)
  • Contract addresses: must be 56-char base32 starting with C
  • DEFAULT_STELLAR_CONFIG (testnet defaults, safe for new
    deployments)

apps/frontend/src/components/app/stellar/useStellarConfigForm.ts —
React hook following the useBrandingForm pattern, with field-level
3. apps/frontend/src/components/app/stellar/StellarConfigPanel.tsx
— Accessible form component with:

• Radio-group network selector with auto-fill of default URLs on
  switch
  • URL inputs with aria-describedby help text and aria-invalid
    error states
• Asset pair list (add/remove, per-side asset fields with
  contextual help)
  • Contract address list (add/remove with inline validation)

4. apps/frontend/src/components/app/stellar/index.ts — barrel
   export
5. apps/frontend/src/lib/customization/validate-stellar.test.ts —
   32 tests, all passing.

closes #18

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[temma02]

Merged manually with conflict resolution (accepting incoming changes).