Feature Request: Open thread in separate window (like Slack "Open in new window")

## Problem

When working in a busy Matrix room with multiple active threads, there is no way to keep a thread visible while working in another part of the app. The thread panel closes or gets buried under other navigation.

ElementX on macOS (and Element Desktop) has no equivalent of Slack's "Open in new window" for individual threads.

## Expected Behaviour

Right-click (or context menu) on a thread should offer "Open thread in new window", spawning a dedicated window showing that thread's full conversation history and composer -- independent of the main window's navigation state.

Pop-out windows should:
- Stay live (push notifications, read receipts, typing indicators)
- Support reply/React in the pop-out
- Close independently without losing the thread in the main app

## Why This Matters

- Multitasking: monitor a hot thread while scanning room history or other threads.
- Multi-monitor setups: put an active thread on one screen, the main room feed on another.
- Parity with Slack/Telegram Desktop which already support this pattern.

## Related

PR #12136 added "Pop out of Threads Activity Centre" -- that pops out the threads panel sidebar, not an individual thread conversation into its own window. This request is a superset: pop out a single thread.

## Environment

- Element Desktop (Electron) / ElementX macOS
- Self-hosted Synapse homeserver

Start by reviewing PR #12136, which added the related Threads Activity Centre pop-out, and trace the existing thread and window behavior from there. Done means a single thread can open in an independent window with its full history and composer, remain live for notifications, read receipts and typing indicators, support replies and reactions, and close without affecting the main window.

Written by the indexing model from the issue text.