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/style.css

@@ -1173,16 +1173,9 @@ input.switch:checked::after { transform: translateX(18px); }
   fill: none;
   opacity: 0.85;
 }
-.chart-svg .wtrend-goal {
-  stroke: var(--walk);
-  stroke-width: 1.5;
-  stroke-dasharray: 2 3;
-  opacity: 0.55;
-}
 .lg.wtrend-today .sw { background: var(--walk); }
 .lg.wtrend-yesterday .sw { background: var(--eat); }
 .lg.wtrend-avg .sw { background: var(--weight); }
-.lg.wtrend-goal .sw { background: color-mix(in srgb, var(--walk) 45%, var(--surface)); }
 .lg[hidden] { display: none; }
 
 .chart-svg .hm-cell { stroke: none; }