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.

audio.py

@@ -136,7 +136,7 @@ class Recorder(QObject):
 
         frames = len(pcm) // (SAMPLE_WIDTH * CHANNELS)
         if frames < MIN_FRAMES:  # a stray keypress, not speech
-            self.failed.emit(t("Recording too short — speak for at least 0.3 s"))
+            self.failed.emit(t("Recording too short, speak for at least 0.3 s"))
             return
 
         path = write_wav(pcm)