Message Display settings and enhanced rendering control

Settings now has a Message Display section for how chat responses are streamed and rendered.

• Stream AI responses: Stream replies word by word with a typing animation. Cost estimation is less accurate while streaming is enabled.

• Enhanced Rendering Capabilities: Control whether the AI is prompted to use extra rendering features, such as Mermaid diagrams, when they would help. Turn this off if you don't want them added to responses.

• Long chats: Enable lazy rendering and pagination, and set how many messages to show per page.

• Text selection toolbar: Show a floating toolbar with extra actions when text is selected in the conversation, and choose its position.