Show the age counter as weeks with a months breakdown

Weeks and calendar months disagreed at a glance ("16 weeks" but only
"3 months"). Now the counter reads "16 weeks (3 months and 3 weeks)
old" so the two units line up, and drops the weeks past 4 months to
show just the months form.

src/changelog.json

@@ -1,4 +1,5 @@
 [
+  { "date": "2026-07-24", "text": "The age counter reads \"16 weeks (3 months and 3 weeks) old\" so weeks and months line up; past 4 months it drops the weeks and shows just months (e.g. \"5 months and 2 weeks old\")" },
   { "date": "2026-07-17", "text": "The Sleep trend chart follows the selected day — pick a past day to see its full curve against the day before and the average leading up to it" },
   { "date": "2026-07-15", "text": "The Sleep trend y-axis is stretched above 10h, giving the hours around the sleep goal most of the chart" },
   { "date": "2026-07-15", "text": "The Sleep trend chart shows the sleep goal for your puppy's age as a shaded band — the projection chip gets a ✓ when today is on track" },