Split view

### Feature summary

Split view for multiple sessions

### What problem are you trying to solve?

Right now the Copilot app only lets me have one chat session open at a time. I can switch between sessions, but that always takes a couple of clicks and breaks my flow a bit.
In practice, I rarely work in just one context. In the terminal I usually have 3–4 panes open at the same time, each doing something different — different tasks, different prompts, sometimes even different models. Being able to see those side by side is a big part of how I work.

With the app, I’m forced into a single session view, which makes it harder to:

- work on multiple things in parallel
- compare responses
- keep different contexts separate

It would be great if there was a way to open multiple Copilot sessions next to each other, similar to split panes in a terminal or editor.
What I’m hoping for:

- open multiple chat sessions side by side (horizontal/vertical split)
- each keeps its own context
- easy way to create a new session in a split
- resize panes / adjust layout

Optional but nice:

- keyboard shortcuts to move between sessions (like a terminal)

This would make Copilot feel much closer to how dev workflows actually work day-to-day, especially when juggling multiple tasks or experiments at once.

### Proposed solution

Split view

### Workflow impact

- Much less clicks and cognitive overhead

### Installation context

_No response_

### Additional context

_No response_

Start by mapping the current single-session view and session creation flow described in the issue. Define how horizontal or vertical splits, independent session context, pane resizing, and creating a session in a split should work; treat keyboard navigation as optional. Done means multiple sessions can be viewed and managed side by side without losing their separate contexts.

Vom Indexierungsmodell aus dem Issue-Text verfasst.