Fullscreen renderer upsell redesigned as an interactive dialog

The passive inline tip promoting the fullscreen renderer has been replaced with a proper interactive dialog that asks "Try the new fullscreen renderer?" and presents "Yes, try it" / "Not now" buttons. Accepting the dialog immediately switches the active session to fullscreen mode.

The tip text has also been updated from "Try smoother rendering, lower memory usage, mouse support, and better formatting of copied text" to "Try the new fullscreen renderer — flicker-free output, mouse support, auto-copy on select", with a context-aware note "— fixes the flashing you see during long responses" added when the classic renderer is detected.