Issue: Collapse + group background for vertical tab groups (follow-up to #9875)

Title: Collapse + group background for vertical tab groups (follow-up to #9875)

Labels to request: enhancement, area:window-tabs-panes, area:settings-keybindings

Problem

#9875 introduces project-grouped vertical tabs but explicitly scopes out collapsible groups. With many tabs across several projects, the vertical tab list grows long and users have no way to fold inactive project sections. There's also no visual treatment that distinguishes one group's tabs from another beyond the section header — when the list scrolls past the header, tabs blur together.

Desired behavior

• Collapse / expand project section headers in the vertical tabs list
  • Click header to fold/unfold
  • Folded groups show only the header + tab count
  • Keyboard shortcut to fold/unfold the focused group
• Group background — subtle accent on tabs belonging to the same group, so the visual grouping persists when the section header is offscreen
  • Reuses existing tab/pane color tokens
  • Default off; opt-in via the same setting that enables Group vertical tabs by project or repo root #9875's project grouping

Scope

• Vertical tabs only (matching Group vertical tabs by project or repo root #9875).
• Default off.
• Collapsed/expanded state persists across restarts.
• Preserves drag/drop semantics (can't drop into a collapsed group without expanding).

Non-goals

• Manual user-defined groups (covered by broader Pinned / Locked tabs and tab groups like in Safari/Chrome #2314).
• Per-group naming or icon (out of scope; project root already names the group).
• Pinned/locked tabs (Pinned / Locked tabs and tab groups like in Safari/Chrome #2314).
• Workspace save/restore (separate follow-up — see Feature: Multi-root Workspace — Add multiple folders to a single workspace (like VS Code) #8875).

Relationship to existing issues

• Group vertical tabs by project or repo root #9875 (open, in flight as Group vertical tabs by project #9876) — introduces the project-grouping primitive this builds on. Lists "Collapsible groups" as an explicit non-goal.
• Group sidebar tabs into collapsible Groups #9469 (closed as duplicate) — covered the same collapse + visual treatment ask but was duped to Pinned / Locked tabs and tab groups like in Safari/Chrome #2314 and Add tab groups #9131.
• Pinned / Locked tabs and tab groups like in Safari/Chrome #2314 (open) — broader: pinned/locked tabs + browser-style groups. Narrower scope here keeps review focused; Pinned / Locked tabs and tab groups like in Safari/Chrome #2314 should remain open for the broader feature set.
• Add tab groups #9131 (closed) — older tab groups umbrella.

@oss-maintainers

I have a focused implementation ready (built on top of the #9876 grouping work in my fork). Would you prefer this to go through a spec PR first, or is this small enough to mark ready-to-implement? Happy to convert to spec if preferred.