feat(composer): localized rich-text editor placeholders + AI button/expand gating

Thread PresentationStrings into RichTextEditorChatInputNode so the composer editor's placeholders come from the new RichText.Placeholder{Quote,Code} strings. Gate the AI button on non-whitespace input content and the expand-input button on isAIEnabled, in the same input panel.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>