fix(notify): group counters instead of padding columns

Pushover renders the notification body in a proportional font, so the
space-padded label/value columns never lined up on a phone — the body
collapsed into a ragged flat list with no visual grouping. The html
variant was worse: the padding sat inside the HTML, so it collapsed to a
single space.

Counters are now grouped two per line and joined with a middle dot, which
survives any font. The body drops from 8 lines to 5 and no longer restates
the outcome the title already carries.

- replace _notify_row/LABEL_WIDTH/VALUE_WIDTH with _stat_row
- add docs/notify-preview.py, which renders the real build_summary()
  output as a phone-width PNG so layout changes can be reviewed visually
- tests: assert no space padding, two counters per row, and that the body
  does not repeat the title

config.toml.example

@@ -26,7 +26,9 @@ use_pushover = false
 po_key = "<key>"
 po_token = "<token>"
 # Notification formatting
-# format: "plain" (aligned text) or "html" (bold labels, Pushover HTML)
+# format: "plain" (grouped text) or "html" (bold labels, Pushover HTML)
+# Counters are grouped two per line and joined with a middle dot, because
+# Pushover uses a proportional font and padded columns do not align.
 format = "plain"
 use_emoji = false
 include_hostname = false