feat(mail): keep mu4e's main view from deleting the window split

mu4e's main view displays with a display-buffer-full-frame action, which tears down every other window on launch, so opening mu4e from a split collapsed the layout. mu4e's own mu4e-display-buffer docstring points to display-buffer-alist as the supported override.

I added an entry routing the *mu4e-main* buffer to the current window (reuse a window already showing it first, then same-window), so launching mu4e in a split keeps the rest of the layout intact. It's registered eagerly rather than inside mu4e's deferred config so it applies on the first launch. Tests cover the entry registration and that the main buffer no longer collapses a split under mu4e's full-frame action.

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