Remove the walking goal

It was mine, not the user's: added unprompted alongside the Walks panel because
sleepTargetFor existed and it seemed to fit. The number came from the
"five-minute rule" — five minutes per month of age, twice a day — written from
general knowledge with no source consulted and none cited in the code.

That rule is widely repeated but is folk guidance, not a veterinary standard.
Its origin is unclear, it is contested by what evidence exists on early
exercise and joint development, and it ignores breed, terrain, lead or off-lead
and the individual dog. Drawing it as a line labelled "Goal", with a ✓ for
clearing it, stated it far more confidently than a rule of thumb earns —
particularly in an app whose entire job is otherwise to record what happened
rather than to advise.

So the target line goes, along with its legend chip, the ✓, and walkTargetFor
itself, which had no other caller. The trend keeps today against yesterday and
the average: a record, with the judgement left to whoever knows the dog.

src/app.js

@@ -664,19 +664,6 @@
       .map(w => ({ start: w.start, end: w.end, ongoing: w.ongoing && today }));
   }
 
-  // Rough age-based walking guideline (the widely used "five-minute rule"):
-  // about 5 minutes per month of age per walk, twice a day, until the puppy is
-  // grown. Returns null without a birthday or once it's a year old, the same
-  // way sleepTargetFor bows out.
-  function walkTargetFor(birthday) {
-    const a = ageParts(birthday);
-    if (!a || a.months >= 12) return null;
-    // Under a month of counted age the rule has nothing to say yet; treat it
-    // as one "month" so the advice stays a short outing rather than zero.
-    const months = Math.max(1, a.months);
-    return { perWalk: months * 5, walks: 2, total: months * 10 };
-  }
-
   // ---------- rendering ----------
   const dayPicker  = document.getElementById("day-picker");
   const eventList  = document.getElementById("event-list");
@@ -1762,7 +1749,7 @@
     };
   }
 
-  function drawWalkTrendChart(curves, target) {
+  function drawWalkTrendChart(curves) {
     const svg = document.getElementById("chart-walk-trend");
     if (!svg) return;
     const W = 320, H = 180;
@@ -1777,12 +1764,7 @@
       { pts: curves.today,     cls: "wtrend-today",     label: curves.dayLabel },
     ].filter(s => s.pts && s.pts.length > 1);
 
-    // The axis has to reach the goal even on a day that fell well short of it,
-    // or the line it is measured against would sit off the top of the chart.
-    const rawMax = Math.max(
-      target ? target.total : 0,
-      ...series.flatMap(s => s.pts.map(p => p.y)),
-    );
+    const rawMax = Math.max(...series.flatMap(s => s.pts.map(p => p.y)));
     const { yMax, steps } = niceAxisLinear(rawMax);
     // Minutes are linear all the way down, so no split axis: the sleep trend
     // stretches its top because the interesting hours cluster near a 16h goal,
@@ -1804,16 +1786,6 @@
       parts.push(`<text x="${x.toFixed(1)}" y="${H - MB + 14}" text-anchor="${anchor}">${pad2(hr)}</text>`);
     }
 
-    // The five-minute rule as one line rather than a band: it is a target to
-    // reach, with no upper bound a band would imply.
-    if (target) {
-      const y = yOf(Math.min(target.total, yMax));
-      parts.push(
-        `<line class="wtrend-goal" x1="${ML}" y1="${y.toFixed(1)}" x2="${W - MR}" y2="${y.toFixed(1)}">` +
-        `<title>${escapeText(`Goal ~${target.total} min (${target.perWalk} min × ${target.walks})`)}</title></line>`
-      );
-    }
-
     for (const s of series) {
       const d = s.pts
         .map((p, i) => `${i === 0 ? "M" : "L"}${xOf(p.x).toFixed(1)} ${yOf(p.y).toFixed(1)}`)
@@ -1830,22 +1802,11 @@
 
   function renderWalkTrend(events) {
     const curves = walkTrendCurves(events);
-    const target = walkTargetFor(loadConfig().birthday);
-    drawWalkTrendChart(curves, target);
+    drawWalkTrendChart(curves);
 
     const chip = (id) => document.getElementById(id);
     const mins = (pts) => `${Math.round(pts[pts.length - 1].y)} min`;
-    // With no projection to carry it, the ✓ goes on today's own chip: the goal
-    // is a total for the day, so today's line is what meets it or doesn't.
-    let mark = "";
-    if (target) {
-      const done = curves.today[curves.today.length - 1].y >= target.total;
-      mark = done ? " ✓" : "";
-      chip("legend-wtrend-today-text").parentElement.title = done
-        ? "Past the walking goal for this age"
-        : `Goal is about ${target.total} min a day at this age`;
-    }
-    chip("legend-wtrend-today-text").textContent = `${curves.dayLabel} ${mins(curves.today)}${mark}`;
+    chip("legend-wtrend-today-text").textContent = `${curves.dayLabel} ${mins(curves.today)}`;
 
     const yLegend = chip("legend-wtrend-yesterday");
     yLegend.hidden = !curves.yesterday;
@@ -1857,11 +1818,6 @@
     if (curves.avg) {
       chip("legend-wtrend-avg-text").textContent = `${curves.avgDays}-day avg ${mins(curves.avg)}`;
     }
-    const gLegend = chip("legend-wtrend-goal");
-    gLegend.hidden = !target;
-    if (target) {
-      chip("legend-wtrend-goal-text").textContent = `Goal ~${target.total} min`;
-    }
   }
 
   // Which heatmap block is focused, remembered across re-renders so a background