Polish AI prompt controls

[roncodes]

Summary

• Removed the redundant End chat button from the AI prompt controls.
• Removed the unused prompt component endSession action.
• Centered and softened the scroll-to-latest button styling.

Validation

• ./node_modules/.bin/eslint addon/components/ai-prompt.js
• ./node_modules/.bin/ember-template-lint addon/components/ai-prompt.hbs (exited successfully; printed existing no-down-event-binding config warning)
• ./node_modules/.bin/stylelint addon/styles/ai-engine.css
• git diff --cached --check