feat(api-keys): show assigned account availability in picker

[stemirkhan]

Summary

• show status, plan, and remaining 5h/7d availability in the API key edit dialog's Assigned accounts picker
• keep the picker concise by omitting model-specific additional quota badges
• add frontend test coverage and an OpenSpec change for the new picker behavior

Why

Operators can already scope an API key to assigned accounts, but the picker only showed identity before this change. Surfacing the base 5h/7d availability in the picker makes account selection faster without forcing a context switch back to Dashboard or Accounts.

Screenshot

Testing

• cd frontend && npx -y node@20 ./node_modules/vitest/vitest.mjs run src/features/api-keys/components/account-multi-select.test.tsx src/features/api-keys/components/api-key-edit-dialog.test.tsx
• export PATH=/home/temirkhan/.local/bin:$PATH && openspec validate --specs

Notes

• follow-up UX improvement for the Assigned accounts picker introduced in feat: add accounts as pools for api to use #338
• uses existing account summary data; no API contract changes