feat: Tab mode for the stack

[cnvuls]

Summary

• Implemented window stacking and column management via consume-or-expel commands.
• Added dedicated keybinds for dynamic column resizing and vertical window movement within stacks.

Motivation

This change improves workspace ergonomics by enabling a fully keyboard-driven window organization workflow. It allows the user to quickly group related windows into stacks (columns) without breaking focus or using a mouse, which is essential for deep work sessions.

Testing

Verified on Niri 25.11:

1. Ran niri validate — no configuration errors found.
2. Tested Mod+Comma / Mod+Period for consume-or-expel functionality across multiple columns.
3. Verified Mod+U for tabbed-display toggle.

Checklist

• Tested on Niri (or Hyprland if applicable)
• Tested both ii and waffle families for UI changes
• Tested material, aurora, and inir styles for ii changes
• No hardcoded values (colors, fonts, durations use design tokens)
• Config changes synced in Config.qml and defaults/config.json
• Config access uses optional chaining: Config.options?.section?.option ?? default
• IPC functions have explicit return types (: void, : string, etc.)
• Shell restarted after changes: qs kill -c ii; qs -c ii
• Logs checked for errors: qs log -c ii | tail -50
• Lazy-loaded components tested (Settings, overlays)
• No console errors or warnings

Related

Closes #
Fixes #