feat: add funded account onboarding state

[Myart352]

PR: Mobile Funded Account Onboarding State (#330)

Summary

Add onboarding UI states for unfunded or newly created accounts. The app now distinguishes between unknown, checking, unfunded, and funded account states, with appropriate guidance and action restrictions.

Changes

New Files

• src/components/FundingStatusBanner.tsx — Component that renders the appropriate banner for each funding status:
  • unknown/checking: Info banner explaining the check
  • unfunded: Warning card with Friendbot funding button
  • funded: Subtle success indicator (optional showTestnetNotice)
• docs/account-funding-states.md — Documentation of funding and balance state architecture.

Modified Files

• src/types/balance.ts — Added FundingStatus type ('unknown' | 'checking' | 'unfunded' | 'funded').
• src/store/walletStore.ts — Added fundingStatus field, checkFundingStatus() action that calls fetchAccountDetails() to determine if account exists on network.
• app/(tabs)/index.tsx — Replaced old FundButton with FundingStatusBanner that shows proper funding state. Also triggers checkFundingStatus() on mount.
• app/send.tsx — Disables the Send button when account is unfunded, with an explanatory warning banner.
• src/components/index.ts — Exported FundingStatusBanner, FundingStatus.

Acceptance Criteria

• Funding status states are added (unknown, checking, unfunded, funded)
• Unfunded account guidance is shown (Friendbot card)
• Unknown state is handled (info banner)
• Payment actions are disabled where appropriate (send blocked when unfunded)
• SDK assumptions are documented

Testing

• Verified no TypeScript errors in modified files
• Funding status updates reactively when wallet state changes
• Send screen correctly disables when fundingStatus === 'unfunded'

Branch

feat/mobile-three-features

Issue

Closes #330