feat(desktop): open a community in a separate window

## Problem

Users who belong to multiple communities have to switch the active community
repeatedly when monitoring or participating in conversations across them.

This makes workflows involving two active communities unnecessarily disruptive,
especially when both need to remain visible at the same time.

## Proposed UX

Add an "Open in new window" action to the existing right-click context menu for
each community in the community rail.

Selecting it would:

- open that community in a separate desktop window;
- focus the existing window if the community is already open;
- keep each window pinned to its own community;
- allow both communities to remain visible and usable simultaneously;
- close only the selected secondary window when that window is closed.

## Important correctness requirement

Operations initiated from each window must always remain scoped to that
window's community. Opening or activating one community window must not change
the relay used by another window.

## Architecture question

The frontend can create another Tauri webview, but the Rust backend currently
models the relay as a process-global active workspace.

Would maintainers prefer:

1. a per-window backend community/session context;
2. a more limited secondary-window surface for an initial implementation;
3. another architecture?

I would like to confirm this boundary before implementing it.

## MVP acceptance criteria

1. The community context menu contains "Open in new window".
2. At most one secondary window exists per community.
3. Selecting an already-open community focuses its window.
4. Each window remains independently scoped to its community.
5. Sending messages from two windows cannot cross relay boundaries.
6. Closing a secondary window does not close or hide the main window.
7. Existing single-window behavior is unchanged.
8. The feature works on macOS, Windows, and Linux.

## Duplicate search

Searched issues and PRs for:
- "open in another window"
- "open in new window"
- "multiple windows"
- "multi-window"
- "second window"
- "separate window"
- community + window / context menu / right click

No equivalent proposal found.