Track walks as timed exercise

A walk is a start and an end, so it reuses the shape sleep already has rather
than inventing one: walk-start / walk-end events, paired into windows, with a
trailing unmatched start meaning "out right now". The server stores type as an
opaque string, so nothing there changes and the events ride the existing sync.

Pairing boundaries into windows was written out twice already, once for sleep
and once for its inverse, so this pulls the scan into pairWindows(open, close)
and makes all three callers of it. Same for the latest-boundary lookup behind
currentSleepState, which currentWalkState now shares — including the updatedAt
tie-break, which matters as soon as a start and an end land in the same minute.

They are called walks, not exercise. "Exercise" is already taken by the
training definitions (their own synced collection, and exerciseId on training
events), and two meanings of the word in one app would be worse than the
slightly narrower name.

The day's total leads the overview tile with the count underneath, since the
question is how much exercise the puppy got rather than how many outings it
took. The Walks panel lists the day's windows and carries the total in its
heading so a collapsed panel still answers it, and the daily charts gain a
minutes-per-day bar chart that stays hidden until there is a walk to draw —
the grams chart's rule. The panel also states the five-minute rule for the
puppy's current age, the same way the sleep trend states a goal band.

Walk boundaries answer to the walk state, not the sleep one, so "Walk end" is
disabled with no walk running and stays undimmed mid-walk even while the puppy
is logged asleep.

src/changelog.json

@@ -1,4 +1,5 @@
 [
+  { "date": "2026-08-31", "text": "Added walks: tap 🦮 Walk start when you head out and 🏁 Walk end when you get home, and the time in between is counted as exercise. The pair gets its own row under the sleep buttons. Today's total and the number of walks show in the overview, every walk of the day is listed in a new Walks section (with the age-based rule of thumb of about five minutes per month of age, twice a day), and the daily charts gain a “Walks (minutes)” bar chart once you have logged one. Like sleep, only the boundary that makes sense is tappable — no walk running means Walk end is greyed out" },
   { "date": "2026-08-31", "text": "The Log event buttons are grouped into rows now instead of flowing into one grid: 😴 Sleep start and ⏰ Sleep end side by side on their own row, then 🍽️ Ate / 💧 Pee / 💩 Poo three across, then ⚖️ Weigh-in and 📝 Note. The two halves of a sleep pair can no longer end up split across a wrap, and no button is left stranded alone on the last row" },
   { "date": "2026-08-24", "text": "The timing charts now show the longest gap as well: each bar keeps its solid stretch from the shortest to the typical gap and fades on out to the longest one of the week. To stop the nightly long gap from squashing the daytime range into a sliver, the bars are stretched so the typical gap always sits dead centre — left of the middle is sooner than usual, right of it is longer, in every row" },
   { "date": "2026-08-24", "text": "Each row of the timing panel is now a small chart instead of a number: the band spans the shortest to the typical gap over the last 7 days, and the marker is how long it has been since the last one. Inside the band means there is time yet, off the right-hand end means the puppy is due — and a row with only one event so far says so instead of drawing an empty axis" },