[FEATURE]: Add a persistent left sidebar layout option to the new UI

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

What I want to change

The new layout removed the persistent left sidebar that showed projects -> workspaces -> sessions. Please keep a layout with an always-visible left sidebar available as an option -- a legacy / classic toggle or a config flag is enough. It does not have to be the default.

If the old layout can't be maintained long-term, then at minimum please restore an equivalent overview inside the new layout: a sidebar that can be pinned open, grouping sessions by project/workspace, with full (non-truncated) session titles instead of only top tabs.

Why this matters

I normally keep many sessions open across several projects at the same time. The sidebar gave me:

• a one-glance view of which project and workspace each session belongs to;
• • one-click context switching without leaving the current session;
• • • a scannable vertical list where session titles stayed readable.
      With the current design the session list is flattened into tabs, titles get truncated, and switching context takes extra clicks or a keyboard shortcut through a single-conversation panel. For anyone running parallel sessions this is a real drop in productivity, not just a cosmetic preference.

Related discussions

• #37012 keep legacy layout option
• • #37273 allow the project/session sidebar alongside tabs
• • • #48882 restore the legacy UI with the persistent left sidebar as an option
• • • • #48837 forced V2 interface hurts multi-project / multi-agent workflows
        I understand the redesign was intentional. Keeping the sidebar as an option would resolve this without forcing every user onto one layout.