fix: wrap getContract in try-catch + null guards + user rejection handling (fixes #152)

[jiangyj545]

Summary

Fixes #152 — approveAsset() (and getAllowance()) threw premature errors when contracts failed to resolve (e.g., wrong network, RPC unavailable) before reaching the wallet interaction.

Root Cause

Both functions called getContract() outside the try/catch block. When a contract could not be resolved (wrong chain, network issue), the error propagated as an unhandled rejection instead of a user-friendly toast.

Changes

File: src/api/assets.js

approveAsset():

• Moved getContract() calls inside try/catch
• Added separate null checks for asset contract vs spender contract with specific error messages
• Silent suppression of MetaMask user rejection (ACTION_REJECTED / code 4001)

getAllowance() — also fixed, same vulnerability:

• Wrapped contract resolution in try/catch
• Added null guard to prevent .address access on undefined
• Fails silently (allowance keeps last known value) instead of crashing

Diff Stats

+30 / -12 lines across 1 file

Verification

• npx rollup -c ✅ builds successfully
• No new dependencies
• No behavior change for happy path (contracts resolve correctly)