Vertical tabs and resizable sidebar sections

[giannisCKS]

I would like an option to show tabs vertically in the sidebar instead of across the top.

I have a local implementation with:

• Horizontal or vertical tabs in Settings.
• Any order of Spaces, Tabs, and Agents.
• Independent scrolling for each section.
• Dividers that resize neighboring sections without moving the third section, with sizes saved across restarts.

My fork: giannisCKS/herdr-vertical-tabs. The initial implementation is on the fix/opencode-subagent-lifecycle branch, also visible in closed PR #5387, mixed with an OpenCode fix.

The latest resizing fixes are still local and are not included in the linked PR or branch yet. I have tested the resizing manually and it works; full-suite validation is still unresolved.

Would this fit Herdr? Sharing here for feedback, not requesting an implementation PR exception.