refactor(web): use css icon for prompt generator

[lyzno1]

Summary

• replace the prompt generator SVG component with the equivalent CSS icon span
• keep the glyph decorative and preserve its 16px size and primary color
• remove exactly one prefer-tailwind-icons warning

Dependency

This PR is stacked on #40328 because it preserves the named generator action and its behavior test. It contains only the icon implementation change.

Visual regression review

Please compare before and after:

• generator glyph shape and 16px dimensions
• primary color in light and dark themes
• inline alignment and ActionButton box geometry
• hover, active, and focus-visible states
• surrounding prompt editor layout

No text, event, accessible name, modal state, or component geometry is intentionally changed.

Validation

• focused Vitest: 1/1 passed
• standalone accessibility lint: 0 findings
• focused format/lint/type check: 0 findings
• full pnpm check: 0 errors, 2058 existing warnings
• warning delta from parent: exactly -1
• git diff --check: passed

Rollback

Revert this PR alone to restore the SVG component. The semantic fix and behavior test in #40328 remain intact.

[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.