feat(sub-accounts): mobile sub-account management view

[vipineth]

Parent PRD

#75

What to build

Ensure the sub-account management view built in #77 is fully usable on mobile. On narrow viewports the view should render as a full-screen bottom sheet or drawer rather than a centered modal. Rows must be touch-friendly (adequate tap targets, no horizontal overflow). Create, rename, and transfer actions must all be reachable and functional on mobile.

Also verify that the mobile header's user menu correctly shows the account list and allows switching, matching the desktop behavior already implemented.

Acceptance criteria

• Management view opens as a bottom sheet or full-screen drawer on mobile viewports
• All rows are legible without horizontal scrolling at 390px width
• Create, rename, and transfer actions are accessible and functional on mobile
• Mobile header user menu shows master + sub-accounts and allows switching
• Active account banner in the trading terminal is visible and not obscured by mobile UI elements

Blocked by

• Blocked by feat(sub-accounts): sub-account management view with Trade action #77 (management view must exist before it can be made mobile-friendly)

User stories addressed

• User story 18 (switch accounts from mobile header)
• User story 19 (access management view from mobile)

[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.