[FEATURE]: Restore the legacy UI with the persistent left sidebar as an option

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

The recent sidebar redesign (#20242) replaced the classic two-panel layout
(persistent left sidebar + session panel) with a Codex-style unified view.
For me this is not a visual refresh — it removed the single most important
part of my daily workflow, and there is currently no way to go back.

What I want changed / added:

1. Keep the legacy UI with the persistent left sidebar available as an
   option (a legacy / classic UI toggle or config flag is enough — it
   doesn't have to be the default).
2. If the legacy layout can't be maintained, at minimum restore an
   always-visible left sidebar — not hover-only menus or a collapsed rail
   that requires extra clicks.

Why this matters (the benefit):

I live inside opencode for hours every day. The left sidebar gives me an
at-a-glance, persistent view of my projects, workspaces, and sessions:

• I can see and switch between projects/contexts without breaking flow.
• Sessions are one click away, not buried behind hover or search.
• Context switching stays fast and predictable.

The new interface hides all of this behind a single unified list with
hover/focus interactions. Every session feels slower and more disoriented.
This is not an aesthetic preference — removing the sidebar with no escape
hatch is a functional regression for anyone who relies on it.

Related:

• #4583 asked for a way to go back to the old UI.
• #20242 introduced the sidebar redesign.

Happy to provide screenshots, my opencode version, and platform details if
that helps. Please don't leave those of us who depend on the left sidebar
behind.