Ship full-resolution WebP screenshots, drop em dashes everywhere

The screenshots were downscaled to 430 px wide, which made the UI text
blurry. Restore them at native 1292 px as lossless WebP, which is also
half the size of the original PNGs (72 KB against 155 KB for the largest).

Rewrite every em dash in prose, comments, docstrings and interface strings
as ordinary punctuation.

paste.py

@@ -66,7 +66,7 @@ def ydotool_ready():
 def press(shortcut="ctrl+v", delay=0.12):
     """Press a key combination through ydotool, e.g. 'ctrl+v'."""
     if not ydotool_ready():
-        raise PasteError(t("ydotool not found — cannot paste automatically."))
+        raise PasteError(t("ydotool not found, cannot paste automatically."))
 
     codes = []
     for key in (k.strip().lower() for k in shortcut.split("+") if k.strip()):