feat: i3-style tabbed window containers

[tellur1an]

Summary

Add i3-style tabbed containers: a per-container layout mode where multiple windows share one tile's space, only the active window is drawn, and a thin tab bar switches between them. This mirrors i3/sway's layout tabs.

Prior discussion

Raised this previously in Discord with Lemmy, who indicated it was fine and would be implemented. Opening this issue to formalize/track it per the usual process.

Motivation

Lets several related windows (terminals, docs, app instances) share one tile without a permanent split, while still showing what else is grouped there and allowing quick switching.

Proposed shape

• Per-container keybind/umbriel msg action to toggle a container between its current layout and tabbed mode.
• A thin tab bar for tabbed containers, showing window title/icon per window, active one highlighted, consistent with existing theming.
• Click-to-switch on tab entries, plus a keybind to cycle next/prev tab.
• Config keys for tab bar position/height/styling, documented under docs/user/.

Happy to help test or provide more detail on i3/sway's behavior if useful.

[Gelmo]

I would also like to see this. I've been on i3 for at least 10 years and am only willing to consider WMs that allow me to maintain most of the workflow I've grown accustom to with i3. Right now, Sway and Niri are the only options I have in Wayland; I'm not interested in hyprland.

[arkonger]

I feel this would be especially helpful for master layout, where the stack gets to be pretty cramped at 3+ windows.