All screens: drop gray text, matching the Journey fix

The epd4in0e panel's palette has no gray at all (just black/white/
yellow/red/blue/green -- see epd4in0e.py's putpalette), so any gray
RGB fill gets Floyd-Steinberg dithered into a black/white checkerboard
across the whole glyph body. At the small font sizes most of these
screens use (Navidrome's artist/album/genre/source lines, Departures'
direction/platform columns, Weather's description/forecast labels,
Clock's date line, the placeholder messages), that reads as blurry,
half-rendered text -- same root cause already fixed in Journey.

Switched every gray fill/outline to solid black across navidrome.py,
weather.py, departures.py, clock.py, and picture.py. Left the
intentionally-faint row-divider hairlines (228,228,228) alone --
those are meant to almost disappear, not readable text. Removed
departures.py's now-unused MUTED constant.

screens/picture.py

@@ -70,7 +70,7 @@ class PictureScreen(Screen):
             font = load_font(size=int(min(width, height) * 0.06))
             msg = "Kein Bild konfiguriert"
             mw = text_width(draw, msg, font)
-            draw.text(((width - mw) / 2, height / 2), msg, font=font, fill=(140, 140, 140))
+            draw.text(((width - mw) / 2, height / 2), msg, font=font, fill="black")
             return image
 
         if settings.get("fit", "cover") == "contain":