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.

filetranscribe.py

@@ -157,7 +157,7 @@ def _to_wav(path, workdir):
 
 
 def _split(wav_path, workdir):
-    """[(chunk path, offset in seconds)] — a single entry for short files."""
+    """[(chunk path, offset in seconds)], a single entry for short files."""
     with contextlib.closing(wave.open(wav_path, "rb")) as src:
         rate = src.getframerate()
         total = src.getnframes()