Added token allowance and balace precheck

[ayandipe]

Close #459

Title: feat: precheck balance before fund_escrow transfer

• Summary: Add an explicit balance pre-check and a clear InsufficientBalance contract error before transferring tokens into escrow. This prevents opaque token panic errors and keeps the Created -> Funded transition atomic.

• Why: Previously fund_escrow called the token transfer directly; token-side failures surfaced as opaque panics. We now surface a stable contract error when the owner lacks sufficient tokens.

• Changes

  • lib.rs: Added InsufficientBalance #[contracterror] and a precheck of owner token balance in fund_escrow. Ensures no state change occurs unless funds are available. (lib.rs)
  • test.rs: Added fund_fails_insufficient_balance unit test that asserts fund_escrow returns InsufficientBalance when owner has no balance. (test.rs)
  • README.md: Documented the new InsufficientBalance error and rationale. (README.md)

• Testing

  • Run locally from repo root:

    . $HOME/.cargo/env
    cd contracts/escrow
    cargo test --lib

  • Test summary from my run: 11 passed; 0 failed.

• Behavior & compatibility

  • Adds numeric error code InsufficientBalance = 10 (stable contract error).
  • Keeps Created -> Funded atomic: balance is checked before any state mutation.
  • No change to refund/release/dispute semantics.
  • Note: I intentionally removed an allowance precheck to avoid breaking owner-authenticated owner->contract transfers in the test environment (owner-auth calls are allowed to transfer without prior explicit allowance in tests). If you want an allowance check for non-owner scenarios, we can add it later with clear semantics.

• Docs

  • README.md updated to include InsufficientBalance and the rationale for prechecking before transfer.

• Commit message

  • feat: precheck balance before fund_escrow transfer

• Next steps (optional)

  • Update backend normalizeContractError mapping to handle the new error code if needed.
  • Push branch and open PR (I can do this for you).

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