🐛 Fix: Approve CAP error shows before wallet confirmation

[chinaxiao123]

Fixes #32

Problem

When clicking "Approve CAP" on the Stake modal, an error was displayed before the wallet (MetaMask) confirmation. This was caused by:

1. Missing loading state — The Approve button had no isSubmitting state, so users saw no feedback during wallet interaction
2. No silent handling of user rejection — When users rejected the MetaMask prompt, the raw error was shown as a modal
3. No allowance refresh — After successful approval, the allowance wasn't refreshed immediately

Changes

src/api/assets.js — approveAsset()

• Added silent handling for user denied / user rejected / user cancelled errors (no error popup when user cancels in MetaMask)

src/components/modals/StakeCAP.svelte — _approveAsset()

• Added isSubmitting = true before calling approveAsset
• Added isSubmitting = false after completion
• Added allowance refresh (checkAllowance()) after successful approval
• Added loading state to the Approve CAP button (isLoading={isSubmitting})

Testing

1. Open Stake CAP modal
2. Click "Approve CAP" → Button shows loading state
3. Confirm in MetaMask → Allowance updates, "Stake" button appears
4. Click "Approve CAP" → Click Cancel in MetaMask → No error shown, button resets