Feat/mobile three features

[Myart352]

PR: Mobile Balance Unavailable State (#329)

Summary

Add a clear balance unavailable state that distinguishes between loading, unavailable (network error), zero balance, and known balance. This prevents misleading users with stale or missing data shown as "0 XLM".

Changes

New Files

• src/types/balance.ts — BalanceState type ('idle' | 'loading' | 'available' | 'unavailable'), BalanceStateCopy interface, and describeBalanceState() for user-facing text.
• src/components/BalanceDisplay.tsx — Component that renders the balance card with full state support:
  • idle/loading: Activity spinner with descriptive text
  • available (positive): Bold balance with public key and last-refreshed timestamp
  • available (zero): Muted balance with zero-balance banner explaining next steps
  • unavailable: Warning-styled card with error message and Retry button
• docs/account-funding-states.md — Documentation of balance and funding state architecture.

Modified Files

• src/store/walletStore.ts — Added balanceState field. refreshWalletData() sets balanceState: 'loading' before fetch, 'available' on success, 'unavailable' on error.
• app/(tabs)/index.tsx — Replaced old static balance card with BalanceDisplay component. Retry calls both refreshWalletData() and checkFundingStatus().
• src/components/index.ts — Exported BalanceDisplay, BalanceDisplayProps.

Acceptance Criteria

• Unavailable balance state is added
• Zero balance remains distinct from unavailable
• Retry action is available (Retry button + pull-to-refresh)
• User-facing copy is clear and safe (no raw error messages)
• Error details are safe

Testing

• Verified no TypeScript errors in modified files
• Balance state transitions correctly: idle → loading → available/unavailable
• Retry action correctly triggers wallet refresh

Branch

feat/mobile-three-features

Issue

Closes #329