feat(sub-accounts): transfer spot tokens between master and sub-account

[vipineth]

Parent PRD

#75

What to build

Extend the transfer modal (built in #81) with a spot token tab. When spot is selected the user sees a token picker (populated from the master's spot balances), an amount input, and the available balance for the selected token on both sides. Submitting calls exchange.subAccountSpotTransfer({ subAccountUser, isDeposit, token, amount }) where token is the Hyperliquid token identifier string and amount is a decimal string (not in wei).

The modal should retain the direction toggle from the perp tab so the same UX applies.

Acceptance criteria

• Transfer modal has two tabs: "Perp USDC" and "Spot"
• Spot tab shows a token picker listing tokens from the master's spot balances
• Selecting a token shows the available balance on the source side
• Amount input accepts a decimal string
• Submitting with 0 or amount exceeding available balance shows a validation error
• Submitting calls subAccountSpotTransfer with the correct token identifier and amount
• Loading/success/error states behave the same as the perp tab

Blocked by

• Blocked by feat(sub-accounts): transfer perp USDC between master and sub-account #81 (perp transfer modal must exist as the base)

User stories addressed

• User story 7 (transfer spot tokens from master to sub)
• User story 8 (transfer spot tokens from sub to master)
• User story 9 (modal shows available balance for both accounts)

[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.