Default to the system monospace font

The default was Berkeley Mono, which is one machine's licensed font and
reads as configuration rather than a default. It is now the system's
monospace font, so a fresh install looks right anywhere and --font is
there for anyone who wants their own.

Getting "the system monospace font" is less direct than it sounds.
Family::Name("monospace") resolves to Noto Sans — not monospaced at all.
cosmic-text's generic Family::Monospace goes through fontdb's built-in
preference, "FreeMono", which is usually absent, and then lands on an
arbitrary face (Adwaita Mono here). Enabling cosmic-text's fontconfig
feature changes nothing, because fontdb's config parser does not pick up
the alias files a distribution actually ships.

So fontconfig is asked directly: `fc-match -f %{family} monospace`, which
is the same answer every other application on the system gets — Noto Sans
Mono here. It costs 10ms on the worker thread that is already waiting for
the compositor, so nothing in wall clock. Without the fontconfig tools a
short list of common distribution defaults is tried instead.

Weight and stretch overrides went too: they were tuned for Berkeley Mono
Medium SemiCondensed, and a family name carries that anyway — the full
name resolves to exactly that face, verified by rendering both.

--verbose now reports the family the labels were shaped with, since the
default legitimately differs from machine to machine.

src/app.rs

@@ -221,7 +221,7 @@ impl App {
         }
         eprintln!(
             "wl-pick: {} tile(s), {} captured; grid {}x{}, surface {}x{} logical \
-             at scale {}, {} MB of capture buffers",
+             at scale {}, {} MB of capture buffers, labels in {:?}",
             self.tiles.len(),
             self.tiles.iter().filter(|t| t.ready).count(),
             self.layout.cols,
@@ -230,6 +230,7 @@ impl App {
             self.layout.height,
             self.scale,
             self.stats.pool_bytes >> 20,
+            self.labels.as_ref().map(|l| l.family()).unwrap_or("none"),
         );
     }