feat: add resizable agent sidebar

Issue Description

Currently, the agent sidebar uses a fixed width. While this works, it can feel limiting on different screen sizes—especially on larger displays where there’s unused space, or smaller ones where content can feel cramped.

Proposal

Allow the sidebar to be resizable via a drag handle on its edge.

Expected behavior

• User can drag the sidebar edge to adjust width
• A minimum width is enforced to maintain usability
• A maximum width is enforced (for example, a percentage of the viewport)

Why this helps

• Improves usability across different screen sizes
• Gives users better control over layout
• Makes the chat experience more flexible and comfortable

Implementation notes

I’ve explored a possible implementation that:

• Adds a resize handle to the sidebar
• Updates width during drag interactions
• Uses clamping (min + max constraints)

Happy to open a PR if this direction makes sense

https://github.com/user-attachments/assets/790f1c15-5310-4647-a8cb-214bd03d3774

Start by locating the agent sidebar component and its existing width and layout styles in the nuxt.com frontend. Trace how the sidebar is rendered, then verify the drag interaction, minimum and maximum width constraints, and behavior across viewport sizes. Done means users can resize the sidebar without disrupting the chat layout.