feat(sub-accounts): create sub-account dialog

[vipineth]

Parent PRD

#75

What to build

Add a "Create Sub-Account" button to the sub-account management view. Clicking it opens a dialog with a single text input for the sub-account name. The name must be 1–16 characters (enforced by the Hyperliquid API schema); the UI validates this live and shows a character counter. Submitting calls exchange.createSubAccount({ name }). On success the dialog closes and the sub-account list refreshes to show the new account. On failure the dialog stays open and shows the error message.

Acceptance criteria

• "Create Sub-Account" button is visible in the management view header
• Dialog contains a name input with a live character counter (max 16)
• Submitting with an empty or >16-char name shows a validation error without making an API call
• Submitting a valid name shows a loading state on the button
• On success: dialog closes, list refetches, new sub-account appears in the list
• On API error: dialog stays open, error message is displayed

Blocked by

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

User stories addressed

• User story 2 (create a new sub-account by name)
• User story 20 (loading, success, error feedback on create)
• User story 23 (list refreshes after create)

[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.