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/clock.py

@@ -50,6 +50,6 @@ class ClockScreen(Screen):
         draw.text(((width - tw) / 2, block_top - time_top), time_str, font=time_font, fill="black")
 
         dw = date_right - date_left
-        draw.text(((width - dw) / 2, block_top + time_h + gap - date_top), date_str, font=date_font, fill=(90, 90, 90))
+        draw.text(((width - dw) / 2, block_top + time_h + gap - date_top), date_str, font=date_font, fill="black")
 
         return image