feat(sub-accounts): transfer perp USDC between master and sub-account

[vipineth]

Parent PRD

#75

What to build

Add a "Transfer" action to each sub-account row in the management view. Clicking it opens a transfer modal scoped to that sub-account. The modal contains a direction toggle (Master → Sub-Account / Sub-Account → Master), an amount input (decimal USD string, e.g. "50.00"), and a display of the available perp USDC balance for both the source and destination accounts. Submitting calls exchange.subAccountTransfer({ subAccountUser, isDeposit, usd }) where usd is the dollar amount × 1,000,000 as an integer.

Note: isDeposit: true = master is sending to sub-account; isDeposit: false = sub-account is sending back to master.

Acceptance criteria

• "Transfer" button appears on each sub-account row in the management view
• Transfer modal opens pre-scoped to the selected sub-account
• Direction toggle switches between Master→Sub and Sub→Master
• Available perp USDC balance is shown for the source account and updates when direction changes
• Amount input accepts decimals; the USD value is converted to integer × 1e6 before submission
• Submitting with 0 or an amount exceeding available balance shows a validation error
• Submitting shows a loading state; on success the modal closes and balance values refresh
• On error the modal stays open and shows the error message

Blocked by

• Blocked by feat(sub-accounts): sub-account management view with Trade action #77 (management view must exist)

User stories addressed

• User story 5 (transfer USDC from master to sub)
• User story 6 (transfer USDC from sub to master)
• User story 9 (modal shows available balance for both accounts)
• User story 22 (loading/success/error feedback on transfer)

[vipineth]

Closing as wontfix. Sub-account switching was reverted in 990bff4 — the store, hook, and user-menu wiring no longer exist. This issue's spec assumes those modules. If sub-accounts return, we'll file fresh issues against the current codebase.