Add a macOS Command-W shortcut to close or hide the window

- [x] I searched open and closed issues for duplicates.
- [x] I am using the latest release (v0.6.2).
- [x] This is one request, not several bundled together.

**Closest existing issue:** none found. The only existing issue, #1, is the Renovate Dependency Dashboard and is unrelated.

## Is this new, or an improvement?

New capability — Berd does not currently provide a way to close or hide the window with `Command-W` when the app is on Home.

## The problem, in my terms

On macOS, I expect `Command-W` to dismiss the active window. I found that when a project is open, pressing `Command-W` currently navigates back to Home instead of closing the window.

However, once Berd is already on Home, there is no useful `Command-W` behavior for getting the window out of the way while keeping the application running.

## What I do today

When a project is open, I can press `Command-W` to return to Home. From Home, I must use a pointer-based window control or another UI path to get the window out of the way.

## What I'd like to see

Preferred behavior:

- When a project is open, `Command-W` may continue to return to Home.
- When Berd is already on Home, `Command-W` should close or hide the window while leaving the Berd application running.
- `Command-W` should never quit Berd; quitting should remain a separate action such as `Command-Q`.

An alternative would be to make `Command-W` close or hide the active window regardless of whether a project is open. The important part is that Home should provide a keyboard-only way to dismiss the window without quitting the app.

## Why this belongs in Berd itself

This is core macOS window and keyboard integration. It cannot be provided by a skill, agent, extension, or automation.

## Non-goals

- Do not make `Command-W` quit Berd.
- Do not change the behavior of `Command-Q`.
- Do not require a separate configurable shortcut system for this basic macOS convention.

## Alternatives I considered

Using the window controls works, but it is slower and breaks the standard keyboard workflow expected by macOS users.

## Environment

- Berd version: 0.6.2
- OS: macOS Tahoe 26.5.2
- Hardware: Apple M4 Pro (Apple Silicon)