Sidebar UI Fixes and Improvements

[Cdddo]

Sidebar UI Fixes and Improvements

This commit fixes various issues with the sidebar and provides some polish.

Enhancements:

• Reduce vertical padding on sidebar list and preferences toggles, allowing for a more compact view and fitting more chats.
• Add color to active chat indicator in sidebar chat / task list, to more clearly indicate the active chat
• Better highlighting of chat name items in sidebar list
• Make scrollbar backgrounds transparent, looks nicer and ensures consistency between dark/light modes
• Replace logo icon in sidebar to version without text (the previous one was too small to read, this makes the icon itself more visible, and also changes color for light mode)
• Make sidebar chat list delete buttons only visible on hover, decluttering the UI

Fixes:

• Remove vertical position shift on tab due to thicker border on chat / tasks tabs
• Ensure logo icon z-index is higher than sidebar content when window is narrow
• Made dimensions between chat list and tasks list consistent (chat items are now exactly half task items, all the padding and margins are now consistent, ensuring perfect alignment and no unsightly shifts when changing tabs)