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.

dikte.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python3
-"""Dikte — press Ctrl+Space, talk, press again: transcribe, clean up, paste.
+"""Dikte: press Ctrl+Space, talk, press again to transcribe, clean up and paste.
 
 Usage:
   dikte.py               run in the background (tray icon)
@@ -91,7 +91,7 @@ class Dikte:
         self.menu.addAction(self.quit_action)
 
         self.tray.setContextMenu(self.menu)
-        self.tray.setToolTip(t("Dikte — ready"))
+        self.tray.setToolTip(t("Dikte: ready"))
         self.tray.activated.connect(self._tray_clicked)
         self._set_icon("audio-input-microphone")
 
@@ -110,9 +110,9 @@ class Dikte:
     def _set_state(self, state):
         self.state = state
         labels = {
-            IDLE: ("Start recording", "audio-input-microphone", "Dikte — ready"),
-            RECORDING: ("Stop and transcribe", "media-record", "Dikte — recording"),
-            BUSY: ("Working…", "view-refresh", "Dikte — working"),
+            IDLE: ("Start recording", "audio-input-microphone", "Dikte: ready"),
+            RECORDING: ("Stop and transcribe", "media-record", "Dikte: recording"),
+            BUSY: ("Working…", "view-refresh", "Dikte: working"),
         }
         label, icon, tip = labels[state]
         self.toggle_action.setText(t(label))