Shorten the weigh-in label to "Weight"

"Weigh-in" was the longest label in the app and, since it moved into a
third-width button, close enough to the edge to wrap on a mid-size phone.
"Weight" clears it with room to spare and loses nothing: the section, the
chart and the dialog field are all called Weight already.

EVENT_LABELS follows the button rather than only the button changing. Every
type's button label and its log label are the same string today, and weight
would have been the one exception — "⚖️ Weight" to log it, "Weigh-in" in the
history row and "Log Weigh-in" on the dialog. The prose that says "weigh-ins"
as a plural noun is left alone; it reads as English, not as a label.

The changelog entry for the rows is amended again rather than gaining a
neighbour, for the same reason as last time: it is one description of one
panel, and the rename is part of what that panel now looks like.

src/app.js

@@ -21,7 +21,7 @@
     "eat":         "Ate",
     "pee":         "Pee",
     "poo":         "Poo",
-    "weight":      "Weigh-in",
+    "weight":      "Weight",
     "training":    "Training",
     "note":        "Note",
   };