Hide dialog weight/grams fields on event types that don't use them

The JS already sets .hidden per type, but 'dialog label { display: block }'
outranks the UA [hidden] rule, so the fields rendered anyway. Add the
explicit [hidden] override, matching the pattern used elsewhere in the file.

src/changelog.json

@@ -1,4 +1,5 @@
 [
+  { "date": "2026-07-15", "text": "Weight and amount fields no longer show up on events that don't use them (e.g. editing a pee)" },
   { "date": "2026-07-15", "text": "The date in the top bar is shorter (no year), so the whole bar fits on smaller iPhones" },
   { "date": "2026-07-13", "text": "The big timer is back at the top — scroll past it and it hops into the frozen bar instead" },
   { "date": "2026-07-13", "text": "The top bar fits on one row: timer, day arrows, date picker and Today" },