Allow closing the window without quitting Berd

- [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 cannot do this today.

## The problem, in my terms

I want to leave Berd running in the background while temporarily getting its window out of the way. Currently, closing the window also exits the application, so hiding the window is the only way to keep Berd running without it being visible.

This makes it difficult to treat Berd as a persistent desktop application that can continue running while its window is closed.

## What I do today

I have to hide the window instead of closing it. There is no way to close the window while keeping Berd and its background services running.

## What I'd like to see

Closing the last Berd window should hide or withdraw the window while leaving the Berd application and its background services running.

There should also be a separate, explicit way to quit the application completely, such as a Quit command.

## Why this belongs in Berd itself

This is a core desktop window-lifecycle behavior. It cannot be provided by a skill, agent, extension, or automation.

## Non-goals

- Do not change an explicit Quit action into a window-hide action.
- Do not require Berd to launch automatically at login.
- Do not require the app to remain running after the user explicitly quits it.

## Alternatives I considered

Using the existing hide behavior works as a temporary workaround, but it is less discoverable and less convenient than being able to close the window while keeping the application running.

## Environment

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

The issue names no files, tests, or entry points. Start by locating Berd's window-close and explicit-quit handling, then verify that closing the last window preserves background services while the explicit Quit action still terminates the application.

Written by the indexing model from the issue text.