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/weather.py

@@ -129,7 +129,7 @@ class WeatherScreen(Screen):
                 settings.get("latitude") and settings.get("longitude")
             ) else "Wetterdaten nicht verfügbar"
             mw = text_width(draw, msg, font)
-            draw.text(((width - mw) / 2, height / 2), msg, font=font, fill=(120, 120, 120))
+            draw.text(((width - mw) / 2, height / 2), msg, font=font, fill="black")
             return image
 
         days = state.get("days") or []
@@ -174,7 +174,7 @@ class WeatherScreen(Screen):
 
         desc_y = temp_y + temp_h + temp_gap
         dw = desc_right - desc_left
-        draw.text(((width - dw) / 2, desc_y - desc_top), desc, font=desc_font, fill=(90, 90, 90))
+        draw.text(((width - dw) / 2, desc_y - desc_top), desc, font=desc_font, fill="black")
 
         if has_forecast:
             forecast_y = current_h
@@ -192,7 +192,7 @@ class WeatherScreen(Screen):
 
                 label = day["label"]
                 lw = text_width(draw, label, label_font)
-                draw.text((cx - lw / 2, y), label, font=label_font, fill=(60, 60, 60))
+                draw.text((cx - lw / 2, y), label, font=label_font, fill="black")
                 y += label_font.size * 1.4
 
                 draw_weather_icon(draw, day["kind"], cx, y + day_icon_size / 2, day_icon_size)
@@ -200,6 +200,6 @@ class WeatherScreen(Screen):
 
                 temps = f"{round(day['high'])}°/{round(day['low'])}°"
                 dtw = text_width(draw, temps, day_temp_font)
-                draw.text((cx - dtw / 2, y), temps, font=day_temp_font, fill=(50, 50, 50))
+                draw.text((cx - dtw / 2, y), temps, font=day_temp_font, fill="black")
 
         return image