refactor(web): use css icon for assigner add

[lyzno1]

Summary

• replace the single decorative Remix add glyph with its matching CSS icon
• preserve the exact 16px size, shrink behavior, and tertiary text color
• remove the now-unused icon import

Boundary

This is an owner-local visual-resource cleanup only. The accessible name remains owned by #40337, and the React component typing cleanup remains in the next PR.

Validation

• focused lint — the one icon-component warning is removed
• pnpm --dir web exec vp test run app/components/workflow/nodes/assigner/__tests__/panel.spec.tsx — 1 passed
• pnpm --dir web lint:a11y app/components/workflow/nodes/assigner/panel.tsx — passed
• pnpm check — 0 errors, 2058 warnings (exactly one fewer)
• git diff --check — passed

Visual regression review

Compare the add glyph shape, 16px bounds, tertiary color, centering, shrink behavior, button box, and hover/active/light/dark states. No visual difference is intended; any mismatch is a regression.

Stack

Depends on #40337. Second PR in stack #40340.

[lyzno1]

Closing during the accessibility stack audit. This layer only changes component typing or icon implementation, adds no user-visible accessibility contract, and carries independent visual/review risk. It can be reconsidered later as a separate cleanup, not as an a11y dependency.