feat: enhance chat UI layout and UX

[07prajwal2000]

Changes

• Extracted \ChatTitleEditor\ to make the chat title editable in-place on conversation pages.
• Adjusted the main chat layout max-width constraints (65% instead of 75%) to keep content focused.
• Perfected topbar positioning to float naturally next to the sidebar toggle button.
• Virtualized the sidebar conversation list using @tanstack/react-virtual\ for infinite scroll performance.