refactor(web): use css icons for chat settings

[lyzno1]

Summary

• replace the two decorative SVG components in the chat-with-history settings popover with matching CSS icons
• preserve the trigger glyph at 18px and the panel glyph at 24px
• remove the now-unused icon imports

Boundary

This PR contains only visual-resource substitutions. The accessible name and state contract remain owned by #40341; the embedded chatbot already uses the matching CSS trigger icon.

Validation

• focused lint — both icon-component warnings removed
• the two focused popover suites — 4 passed
• standalone a11y lint on both production components — passed
• pnpm check — 0 errors, 2057 warnings (exactly two fewer)
• git diff --check — passed

Visual regression review

Compare the chat-settings glyph and filled message glyph shape, 18px/24px bounds, current-color rendering, alignment, shrink behavior, button box, panel spacing, hover/open states, and light/dark themes. No visual difference is intended; any mismatch is a regression.

Stack

Depends on #40341. Final PR in stack #40343.

[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.